Re: [PATCH v3 1/1] virtio-blk: avoid preallocating big SGL for data

Re: [PATCH v3 1/1] virtio-blk: avoid preallocating big SGL for data

[Michael S. Tsirkin]

On Wed, Sep 01, 2021 at 04:14:34PM +0300, Max Gurtovoy wrote:
> No need to pre-allocate a big buffer for the IO SGL anymore. If a device
> has lots of deep queues, preallocation for the sg list can consume
> substantial amounts of memory. For HW virtio-blk device, nr_hw_queues
> can be 64 or 128 and each queue's depth might be 128. This means the
> resulting preallocation for the data SGLs is big.
> 
> Switch to runtime allocation for SGL for lists longer than 2 entries.
> This is the approach used by NVMe drivers so it should be reasonable for
> virtio block as well. Runtime SGL allocation has always been the case
> for the legacy I/O path so this is nothing new.
> 
> The preallocated small SGL depends on SG_CHAIN so if the ARCH doesn't
> support SG_CHAIN, use only runtime allocation for the SGL.
> 
> Re-organize the setup of the IO request to fit the new sg chain
> mechanism.
> 
> No performance degradation was seen (fio libaio engine with 16 jobs and
> 128 iodepth):
> 
> IO size      IOPs Rand Read (before/after)         IOPs Rand Write (before/after)
> --------     ---------------------------------    ----------------------------------
> 512B          318K/316K                                    329K/325K
> 
> 4KB           323K/321K                                    353K/349K
> 
> 16KB          199K/208K                                    250K/275K
> 
> 128KB         36K/36.1K                                    39.2K/41.7K
> 
> Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
> Reviewed-by: Israel Rukshin <israelr@nvidia.com>

Could you use something to give confidence intervals maybe?
As it is it looks like a 1-2% regression for 512B and 4KB.



> ---
> 
> changes from V2:
>  - initialize vbr->out_hdr.sector during virtblk_setup_cmd
> 
> changes from V1:
>  - Kconfig update (from Christoph)
>  - Re-order cmd setup (from Christoph)
>  - use flexible sg pointer in the cmd (from Christoph)
>  - added perf numbers to commit msg (from Feng Li)
> 
> ---
>  drivers/block/Kconfig      |   1 +
>  drivers/block/virtio_blk.c | 155 +++++++++++++++++++++++--------------
>  2 files changed, 100 insertions(+), 56 deletions(-)
> 
> diff --git a/drivers/block/Kconfig b/drivers/block/Kconfig
> index 63056cfd4b62..ca25a122b8ee 100644
> --- a/drivers/block/Kconfig
> +++ b/drivers/block/Kconfig
> @@ -395,6 +395,7 @@ config XEN_BLKDEV_BACKEND
>  config VIRTIO_BLK
>  	tristate "Virtio block driver"
>  	depends on VIRTIO
> +	select SG_POOL
>  	help
>  	  This is the virtual block driver for virtio.  It can be used with
>            QEMU based VMMs (like KVM or Xen).  Say Y or M.
> diff --git a/drivers/block/virtio_blk.c b/drivers/block/virtio_blk.c
> index 9332fc4e9b31..bdd6d415bd20 100644
> --- a/drivers/block/virtio_blk.c
> +++ b/drivers/block/virtio_blk.c
> @@ -24,6 +24,12 @@
>  /* The maximum number of sg elements that fit into a virtqueue */
>  #define VIRTIO_BLK_MAX_SG_ELEMS 32768
>  
> +#ifdef CONFIG_ARCH_NO_SG_CHAIN
> +#define VIRTIO_BLK_INLINE_SG_CNT	0
> +#else
> +#define VIRTIO_BLK_INLINE_SG_CNT	2
> +#endif
> +
>  static int virtblk_queue_count_set(const char *val,
>  		const struct kernel_param *kp)
>  {
> @@ -93,6 +99,7 @@ struct virtio_blk {
>  struct virtblk_req {
>  	struct virtio_blk_outhdr out_hdr;
>  	u8 status;
> +	struct sg_table sg_table;
>  	struct scatterlist sg[];
>  };
>  
> @@ -178,15 +185,94 @@ static int virtblk_setup_discard_write_zeroes(struct request *req, bool unmap)
>  	return 0;
>  }
>  
> -static inline void virtblk_request_done(struct request *req)
> +static void virtblk_unmap_data(struct request *req, struct virtblk_req *vbr)
>  {
> -	struct virtblk_req *vbr = blk_mq_rq_to_pdu(req);
> +	if (blk_rq_nr_phys_segments(req))
> +		sg_free_table_chained(&vbr->sg_table,
> +				      VIRTIO_BLK_INLINE_SG_CNT);
> +}
> +
> +static int virtblk_map_data(struct blk_mq_hw_ctx *hctx, struct request *req,
> +		struct virtblk_req *vbr)
> +{
> +	int err;
> +
> +	if (!blk_rq_nr_phys_segments(req))
> +		return 0;
> +
> +	vbr->sg_table.sgl = vbr->sg;
> +	err = sg_alloc_table_chained(&vbr->sg_table,
> +				     blk_rq_nr_phys_segments(req),
> +				     vbr->sg_table.sgl,
> +				     VIRTIO_BLK_INLINE_SG_CNT);
> +	if (unlikely(err))
> +		return -ENOMEM;
>  
> +	return blk_rq_map_sg(hctx->queue, req, vbr->sg_table.sgl);
> +}
> +
> +static void virtblk_cleanup_cmd(struct request *req)
> +{
>  	if (req->rq_flags & RQF_SPECIAL_PAYLOAD) {
>  		kfree(page_address(req->special_vec.bv_page) +
>  		      req->special_vec.bv_offset);
>  	}
> +}
> +
> +static int virtblk_setup_cmd(struct virtio_device *vdev, struct request *req,
> +		struct virtblk_req *vbr)
> +{
> +	bool unmap = false;
> +	u32 type;
> +
> +	vbr->out_hdr.sector = 0;
> +
> +	switch (req_op(req)) {
> +	case REQ_OP_READ:
> +		type = VIRTIO_BLK_T_IN;
> +		vbr->out_hdr.sector = cpu_to_virtio64(vdev,
> +						      blk_rq_pos(req));
> +		break;
> +	case REQ_OP_WRITE:
> +		type = VIRTIO_BLK_T_OUT;
> +		vbr->out_hdr.sector = cpu_to_virtio64(vdev,
> +						      blk_rq_pos(req));
> +		break;
> +	case REQ_OP_FLUSH:
> +		type = VIRTIO_BLK_T_FLUSH;
> +		break;
> +	case REQ_OP_DISCARD:
> +		type = VIRTIO_BLK_T_DISCARD;
> +		break;
> +	case REQ_OP_WRITE_ZEROES:
> +		type = VIRTIO_BLK_T_WRITE_ZEROES;
> +		unmap = !(req->cmd_flags & REQ_NOUNMAP);
> +		break;
> +	case REQ_OP_DRV_IN:
> +		type = VIRTIO_BLK_T_GET_ID;
> +		break;
> +	default:
> +		WARN_ON_ONCE(1);
> +		return BLK_STS_IOERR;
> +	}
>  
> +	vbr->out_hdr.type = cpu_to_virtio32(vdev, type);
> +	vbr->out_hdr.ioprio = cpu_to_virtio32(vdev, req_get_ioprio(req));
> +
> +	if (type == VIRTIO_BLK_T_DISCARD || type == VIRTIO_BLK_T_WRITE_ZEROES) {
> +		if (virtblk_setup_discard_write_zeroes(req, unmap))
> +			return BLK_STS_RESOURCE;
> +	}
> +
> +	return 0;
> +}
> +
> +static inline void virtblk_request_done(struct request *req)
> +{
> +	struct virtblk_req *vbr = blk_mq_rq_to_pdu(req);
> +
> +	virtblk_unmap_data(req, vbr);
> +	virtblk_cleanup_cmd(req);
>  	blk_mq_end_request(req, virtblk_result(vbr));
>  }
>  
> @@ -244,57 +330,23 @@ static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
>  	int qid = hctx->queue_num;
>  	int err;
>  	bool notify = false;
> -	bool unmap = false;
> -	u32 type;
>  
>  	BUG_ON(req->nr_phys_segments + 2 > vblk->sg_elems);
>  
> -	switch (req_op(req)) {
> -	case REQ_OP_READ:
> -	case REQ_OP_WRITE:
> -		type = 0;
> -		break;
> -	case REQ_OP_FLUSH:
> -		type = VIRTIO_BLK_T_FLUSH;
> -		break;
> -	case REQ_OP_DISCARD:
> -		type = VIRTIO_BLK_T_DISCARD;
> -		break;
> -	case REQ_OP_WRITE_ZEROES:
> -		type = VIRTIO_BLK_T_WRITE_ZEROES;
> -		unmap = !(req->cmd_flags & REQ_NOUNMAP);
> -		break;
> -	case REQ_OP_DRV_IN:
> -		type = VIRTIO_BLK_T_GET_ID;
> -		break;
> -	default:
> -		WARN_ON_ONCE(1);
> -		return BLK_STS_IOERR;
> -	}
> -
> -	vbr->out_hdr.type = cpu_to_virtio32(vblk->vdev, type);
> -	vbr->out_hdr.sector = type ?
> -		0 : cpu_to_virtio64(vblk->vdev, blk_rq_pos(req));
> -	vbr->out_hdr.ioprio = cpu_to_virtio32(vblk->vdev, req_get_ioprio(req));
> +	err = virtblk_setup_cmd(vblk->vdev, req, vbr);
> +	if (unlikely(err))
> +		return err;
>  
>  	blk_mq_start_request(req);
>  
> -	if (type == VIRTIO_BLK_T_DISCARD || type == VIRTIO_BLK_T_WRITE_ZEROES) {
> -		err = virtblk_setup_discard_write_zeroes(req, unmap);
> -		if (err)
> -			return BLK_STS_RESOURCE;
> -	}
> -
> -	num = blk_rq_map_sg(hctx->queue, req, vbr->sg);
> -	if (num) {
> -		if (rq_data_dir(req) == WRITE)
> -			vbr->out_hdr.type |= cpu_to_virtio32(vblk->vdev, VIRTIO_BLK_T_OUT);
> -		else
> -			vbr->out_hdr.type |= cpu_to_virtio32(vblk->vdev, VIRTIO_BLK_T_IN);
> +	num = virtblk_map_data(hctx, req, vbr);
> +	if (unlikely(num < 0)) {
> +		virtblk_cleanup_cmd(req);
> +		return BLK_STS_RESOURCE;
>  	}
>  
>  	spin_lock_irqsave(&vblk->vqs[qid].lock, flags);
> -	err = virtblk_add_req(vblk->vqs[qid].vq, vbr, vbr->sg, num);
> +	err = virtblk_add_req(vblk->vqs[qid].vq, vbr, vbr->sg_table.sgl, num);
>  	if (err) {
>  		virtqueue_kick(vblk->vqs[qid].vq);
>  		/* Don't stop the queue if -ENOMEM: we may have failed to
> @@ -303,6 +355,8 @@ static blk_status_t virtio_queue_rq(struct blk_mq_hw_ctx *hctx,
>  		if (err == -ENOSPC)
>  			blk_mq_stop_hw_queue(hctx);
>  		spin_unlock_irqrestore(&vblk->vqs[qid].lock, flags);
> +		virtblk_unmap_data(req, vbr);
> +		virtblk_cleanup_cmd(req);
>  		switch (err) {
>  		case -ENOSPC:
>  			return BLK_STS_DEV_RESOURCE;
> @@ -681,16 +735,6 @@ static const struct attribute_group *virtblk_attr_groups[] = {
>  	NULL,
>  };
>  
> -static int virtblk_init_request(struct blk_mq_tag_set *set, struct request *rq,
> -		unsigned int hctx_idx, unsigned int numa_node)
> -{
> -	struct virtio_blk *vblk = set->driver_data;
> -	struct virtblk_req *vbr = blk_mq_rq_to_pdu(rq);
> -
> -	sg_init_table(vbr->sg, vblk->sg_elems);
> -	return 0;
> -}
> -
>  static int virtblk_map_queues(struct blk_mq_tag_set *set)
>  {
>  	struct virtio_blk *vblk = set->driver_data;
> @@ -703,7 +747,6 @@ static const struct blk_mq_ops virtio_mq_ops = {
>  	.queue_rq	= virtio_queue_rq,
>  	.commit_rqs	= virtio_commit_rqs,
>  	.complete	= virtblk_request_done,
> -	.init_request	= virtblk_init_request,
>  	.map_queues	= virtblk_map_queues,
>  };
>  
> @@ -783,7 +826,7 @@ static int virtblk_probe(struct virtio_device *vdev)
>  	vblk->tag_set.flags = BLK_MQ_F_SHOULD_MERGE;
>  	vblk->tag_set.cmd_size =
>  		sizeof(struct virtblk_req) +
> -		sizeof(struct scatterlist) * sg_elems;
> +		sizeof(struct scatterlist) * VIRTIO_BLK_INLINE_SG_CNT;
>  	vblk->tag_set.driver_data = vblk;
>  	vblk->tag_set.nr_hw_queues = vblk->num_vqs;
>  
> -- 
> 2.18.1

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Re: [PATCH v3 1/1] virtio-blk: avoid preallocating big SGL for data

[Jens Axboe]

On 9/1/21 8:58 AM, Max Gurtovoy wrote:
> 
> On 9/1/2021 5:50 PM, Michael S. Tsirkin wrote:
>> On Wed, Sep 01, 2021 at 04:14:34PM +0300, Max Gurtovoy wrote:
>>> No need to pre-allocate a big buffer for the IO SGL anymore. If a device
>>> has lots of deep queues, preallocation for the sg list can consume
>>> substantial amounts of memory. For HW virtio-blk device, nr_hw_queues
>>> can be 64 or 128 and each queue's depth might be 128. This means the
>>> resulting preallocation for the data SGLs is big.
>>>
>>> Switch to runtime allocation for SGL for lists longer than 2 entries.
>>> This is the approach used by NVMe drivers so it should be reasonable for
>>> virtio block as well. Runtime SGL allocation has always been the case
>>> for the legacy I/O path so this is nothing new.
>>>
>>> The preallocated small SGL depends on SG_CHAIN so if the ARCH doesn't
>>> support SG_CHAIN, use only runtime allocation for the SGL.
>>>
>>> Re-organize the setup of the IO request to fit the new sg chain
>>> mechanism.
>>>
>>> No performance degradation was seen (fio libaio engine with 16 jobs and
>>> 128 iodepth):
>>>
>>> IO size      IOPs Rand Read (before/after)         IOPs Rand Write (before/after)
>>> --------     ---------------------------------    ----------------------------------
>>> 512B          318K/316K                                    329K/325K
>>>
>>> 4KB           323K/321K                                    353K/349K
>>>
>>> 16KB          199K/208K                                    250K/275K
>>>
>>> 128KB         36K/36.1K                                    39.2K/41.7K
>>>
>>> Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
>>> Reviewed-by: Israel Rukshin <israelr@nvidia.com>
>> Could you use something to give confidence intervals maybe?
>> As it is it looks like a 1-2% regression for 512B and 4KB.
> 
> 1%-2% is not a regression. It's a device/env/test variance.
> 
> This is just one test results. I run it many times and got difference by 
> +/- 2%-3% in each run for each sides.
> 
> Even if I run same driver without changes I get 2%-3% difference between 
> runs.
> 
> If you have a perf test suite for virtio-blk it will be great if you can 
> run it, or maybe Feng Li has.

You're adding an allocation to the hot path, and a free to the
completion hot path. It's not unreasonable to expect that there could be
performance implications associated with that. Which would be
particularly evident with 1 segment requests, as the results would seem
to indicate as well.

Probably needs better testing. A profile of a peak run before and after
and a diff of the two might also be interesting.

The common idiom for situations like this is to have an inline part that
holds 1-2 segments, and then only punt to alloc if you need more than
that. As the number of segments grows, the cost per request matters
less.

-- 
Jens Axboe

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Re: [PATCH v3 1/1] virtio-blk: avoid preallocating big SGL for data

[Jens Axboe]

On 9/1/21 4:25 PM, Max Gurtovoy wrote:
> 
> On 9/1/2021 6:27 PM, Jens Axboe wrote:
>> On 9/1/21 8:58 AM, Max Gurtovoy wrote:
>>> On 9/1/2021 5:50 PM, Michael S. Tsirkin wrote:
>>>> On Wed, Sep 01, 2021 at 04:14:34PM +0300, Max Gurtovoy wrote:
>>>>> No need to pre-allocate a big buffer for the IO SGL anymore. If a device
>>>>> has lots of deep queues, preallocation for the sg list can consume
>>>>> substantial amounts of memory. For HW virtio-blk device, nr_hw_queues
>>>>> can be 64 or 128 and each queue's depth might be 128. This means the
>>>>> resulting preallocation for the data SGLs is big.
>>>>>
>>>>> Switch to runtime allocation for SGL for lists longer than 2 entries.
>>>>> This is the approach used by NVMe drivers so it should be reasonable for
>>>>> virtio block as well. Runtime SGL allocation has always been the case
>>>>> for the legacy I/O path so this is nothing new.
>>>>>
>>>>> The preallocated small SGL depends on SG_CHAIN so if the ARCH doesn't
>>>>> support SG_CHAIN, use only runtime allocation for the SGL.
>>>>>
>>>>> Re-organize the setup of the IO request to fit the new sg chain
>>>>> mechanism.
>>>>>
>>>>> No performance degradation was seen (fio libaio engine with 16 jobs and
>>>>> 128 iodepth):
>>>>>
>>>>> IO size      IOPs Rand Read (before/after)         IOPs Rand Write (before/after)
>>>>> --------     ---------------------------------    ----------------------------------
>>>>> 512B          318K/316K                                    329K/325K
>>>>>
>>>>> 4KB           323K/321K                                    353K/349K
>>>>>
>>>>> 16KB          199K/208K                                    250K/275K
>>>>>
>>>>> 128KB         36K/36.1K                                    39.2K/41.7K
>>>>>
>>>>> Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
>>>>> Reviewed-by: Israel Rukshin <israelr@nvidia.com>
>>>> Could you use something to give confidence intervals maybe?
>>>> As it is it looks like a 1-2% regression for 512B and 4KB.
>>> 1%-2% is not a regression. It's a device/env/test variance.
>>>
>>> This is just one test results. I run it many times and got difference by
>>> +/- 2%-3% in each run for each sides.
>>>
>>> Even if I run same driver without changes I get 2%-3% difference between
>>> runs.
>>>
>>> If you have a perf test suite for virtio-blk it will be great if you can
>>> run it, or maybe Feng Li has.
>> You're adding an allocation to the hot path, and a free to the
>> completion hot path. It's not unreasonable to expect that there could be
>> performance implications associated with that. Which would be
>> particularly evident with 1 segment requests, as the results would seem
>> to indicate as well.
> 
> but for sg_nents <= 2 there is no dynamic allocation also in this patch 
> exactly as we do in nvmf RDMA and FC for example.

My quick read missed that, which is why you're using chaining. Then it
looks very reasonable to me.

>> Probably needs better testing. A profile of a peak run before and after
>> and a diff of the two might also be interesting.
> 
> I'll run ezfio test suite with stronger virtio-blk device that reach > 
> 800KIOPs

That'd be better, and preferably a test with pinning etc so that you can
show more consistent results. Just reading your table does indeed look
like there's a performance degradation, even if you preface it by saying
there is none. It would need better explaining, but preferably better
testing.

>> The common idiom for situations like this is to have an inline part that
>> holds 1-2 segments, and then only punt to alloc if you need more than
>> that. As the number of segments grows, the cost per request matters
>> less.
> 
> isn't this the case here ? or am I missing something ?

it totally is, I was the one missing that.

-- 
Jens Axboe

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Re: [PATCH v3 1/1] virtio-blk: avoid preallocating big SGL for data

[Stefan Hajnoczi]

[-- Attachment #1.1: Type: text/plain, Size: 2254 bytes --]

On Wed, Sep 01, 2021 at 04:14:34PM +0300, Max Gurtovoy wrote:
> No need to pre-allocate a big buffer for the IO SGL anymore. If a device
> has lots of deep queues, preallocation for the sg list can consume
> substantial amounts of memory. For HW virtio-blk device, nr_hw_queues
> can be 64 or 128 and each queue's depth might be 128. This means the
> resulting preallocation for the data SGLs is big.
> 
> Switch to runtime allocation for SGL for lists longer than 2 entries.
> This is the approach used by NVMe drivers so it should be reasonable for
> virtio block as well. Runtime SGL allocation has always been the case
> for the legacy I/O path so this is nothing new.
> 
> The preallocated small SGL depends on SG_CHAIN so if the ARCH doesn't
> support SG_CHAIN, use only runtime allocation for the SGL.
> 
> Re-organize the setup of the IO request to fit the new sg chain
> mechanism.
> 
> No performance degradation was seen (fio libaio engine with 16 jobs and
> 128 iodepth):
> 
> IO size      IOPs Rand Read (before/after)         IOPs Rand Write (before/after)
> --------     ---------------------------------    ----------------------------------
> 512B          318K/316K                                    329K/325K
> 
> 4KB           323K/321K                                    353K/349K
> 
> 16KB          199K/208K                                    250K/275K
> 
> 128KB         36K/36.1K                                    39.2K/41.7K
> 
> Signed-off-by: Max Gurtovoy <mgurtovoy@nvidia.com>
> Reviewed-by: Israel Rukshin <israelr@nvidia.com>
> ---
> 
> changes from V2:
>  - initialize vbr->out_hdr.sector during virtblk_setup_cmd
> 
> changes from V1:
>  - Kconfig update (from Christoph)
>  - Re-order cmd setup (from Christoph)
>  - use flexible sg pointer in the cmd (from Christoph)
>  - added perf numbers to commit msg (from Feng Li)
> 
> ---
>  drivers/block/Kconfig      |   1 +
>  drivers/block/virtio_blk.c | 155 +++++++++++++++++++++++--------------
>  2 files changed, 100 insertions(+), 56 deletions(-)

Hi Max,
I can run benchmark to give everyone more confidence about this change.
Should I test this version or are you still planning to make code
changes?

Stefan

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Re: [PATCH v3 1/1] virtio-blk: avoid preallocating big SGL for data

[Stefan Hajnoczi]

[-- Attachment #1.1: Type: text/plain, Size: 2325 bytes --]

On Wed, Sep 01, 2021 at 04:14:34PM +0300, Max Gurtovoy wrote:
> No need to pre-allocate a big buffer for the IO SGL anymore. If a device
> has lots of deep queues, preallocation for the sg list can consume
> substantial amounts of memory. For HW virtio-blk device, nr_hw_queues
> can be 64 or 128 and each queue's depth might be 128. This means the
> resulting preallocation for the data SGLs is big.
> 
> Switch to runtime allocation for SGL for lists longer than 2 entries.
> This is the approach used by NVMe drivers so it should be reasonable for
> virtio block as well. Runtime SGL allocation has always been the case
> for the legacy I/O path so this is nothing new.
> 
> The preallocated small SGL depends on SG_CHAIN so if the ARCH doesn't
> support SG_CHAIN, use only runtime allocation for the SGL.
> 
> Re-organize the setup of the IO request to fit the new sg chain
> mechanism.
> 
> No performance degradation was seen (fio libaio engine with 16 jobs and
> 128 iodepth):
> 
> IO size      IOPs Rand Read (before/after)         IOPs Rand Write (before/after)
> --------     ---------------------------------    ----------------------------------
> 512B          318K/316K                                    329K/325K
> 
> 4KB           323K/321K                                    353K/349K
> 
> 16KB          199K/208K                                    250K/275K
> 
> 128KB         36K/36.1K                                    39.2K/41.7K

I ran fio randread benchmarks with 4k, 16k, 64k, and 128k at iodepth 1,
8, and 64 on two vCPUs. The results look fine, there is no significant
regression.

iodepth=1 and iodepth=64 are very consistent. For some reason the
iodepth=8 has significant variance but I don't think it's the fault of
this patch.

Fio results and the Jupyter notebook export are available here (check
out benchmark.html to see the graphs):

https://gitlab.com/stefanha/virt-playbooks/-/tree/virtio-blk-sgl-allocation-benchmark/notebook

Guest:
- Fedora 34
- Linux v5.14
- 2 vCPUs (pinned), 4 GB RAM (single host NUMA node)
- 1 IOThread (pinned)
- virtio-blk aio=native,cache=none,format=raw
- QEMU 6.1.0

Host:
- RHEL 8.3
- Linux 4.18.0-240.22.1.el8_3.x86_64
- Intel(R) Xeon(R) Silver 4214 CPU @ 2.20GHz
- Intel Optane DC P4800X

Stefan

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Re: [PATCH v3 1/1] virtio-blk: avoid preallocating big SGL for data

[Feng Li]

On Mon, Sep 6, 2021 at 11:39 PM Stefan Hajnoczi <stefanha@redhat.com> wrote:
>
> On Wed, Sep 01, 2021 at 04:14:34PM +0300, Max Gurtovoy wrote:
> > No need to pre-allocate a big buffer for the IO SGL anymore. If a device
> > has lots of deep queues, preallocation for the sg list can consume
> > substantial amounts of memory. For HW virtio-blk device, nr_hw_queues
> > can be 64 or 128 and each queue's depth might be 128. This means the
> > resulting preallocation for the data SGLs is big.
> >
> > Switch to runtime allocation for SGL for lists longer than 2 entries.
> > This is the approach used by NVMe drivers so it should be reasonable for
> > virtio block as well. Runtime SGL allocation has always been the case
> > for the legacy I/O path so this is nothing new.
> >
> > The preallocated small SGL depends on SG_CHAIN so if the ARCH doesn't
> > support SG_CHAIN, use only runtime allocation for the SGL.
> >
> > Re-organize the setup of the IO request to fit the new sg chain
> > mechanism.
> >
> > No performance degradation was seen (fio libaio engine with 16 jobs and
> > 128 iodepth):
> >
> > IO size      IOPs Rand Read (before/after)         IOPs Rand Write (before/after)
> > --------     ---------------------------------    ----------------------------------
> > 512B          318K/316K                                    329K/325K
> >
> > 4KB           323K/321K                                    353K/349K
> >
> > 16KB          199K/208K                                    250K/275K
> >
> > 128KB         36K/36.1K                                    39.2K/41.7K
>
> I ran fio randread benchmarks with 4k, 16k, 64k, and 128k at iodepth 1,
> 8, and 64 on two vCPUs. The results look fine, there is no significant
> regression.
>
> iodepth=1 and iodepth=64 are very consistent. For some reason the
> iodepth=8 has significant variance but I don't think it's the fault of
> this patch.
>
> Fio results and the Jupyter notebook export are available here (check
> out benchmark.html to see the graphs):
>
> https://gitlab.com/stefanha/virt-playbooks/-/tree/virtio-blk-sgl-allocation-benchmark/notebook
>
> Guest:
> - Fedora 34
> - Linux v5.14
> - 2 vCPUs (pinned), 4 GB RAM (single host NUMA node)
> - 1 IOThread (pinned)
> - virtio-blk aio=native,cache=none,format=raw
> - QEMU 6.1.0
>
> Host:
> - RHEL 8.3
> - Linux 4.18.0-240.22.1.el8_3.x86_64
> - Intel(R) Xeon(R) Silver 4214 CPU @ 2.20GHz
> - Intel Optane DC P4800X
>
> Stefan

Reviewed-by: Feng Li <lifeng1519@gmail.com>
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Re: [PATCH v3 1/1] virtio-blk: avoid preallocating big SGL for data

[Stefan Hajnoczi]

[-- Attachment #1.1: Type: text/plain, Size: 2951 bytes --]

On Mon, Sep 13, 2021 at 05:50:21PM +0300, Max Gurtovoy wrote:
> 
> On 9/6/2021 6:09 PM, Stefan Hajnoczi wrote:
> > On Wed, Sep 01, 2021 at 04:14:34PM +0300, Max Gurtovoy wrote:
> > > No need to pre-allocate a big buffer for the IO SGL anymore. If a device
> > > has lots of deep queues, preallocation for the sg list can consume
> > > substantial amounts of memory. For HW virtio-blk device, nr_hw_queues
> > > can be 64 or 128 and each queue's depth might be 128. This means the
> > > resulting preallocation for the data SGLs is big.
> > > 
> > > Switch to runtime allocation for SGL for lists longer than 2 entries.
> > > This is the approach used by NVMe drivers so it should be reasonable for
> > > virtio block as well. Runtime SGL allocation has always been the case
> > > for the legacy I/O path so this is nothing new.
> > > 
> > > The preallocated small SGL depends on SG_CHAIN so if the ARCH doesn't
> > > support SG_CHAIN, use only runtime allocation for the SGL.
> > > 
> > > Re-organize the setup of the IO request to fit the new sg chain
> > > mechanism.
> > > 
> > > No performance degradation was seen (fio libaio engine with 16 jobs and
> > > 128 iodepth):
> > > 
> > > IO size      IOPs Rand Read (before/after)         IOPs Rand Write (before/after)
> > > --------     ---------------------------------    ----------------------------------
> > > 512B          318K/316K                                    329K/325K
> > > 
> > > 4KB           323K/321K                                    353K/349K
> > > 
> > > 16KB          199K/208K                                    250K/275K
> > > 
> > > 128KB         36K/36.1K                                    39.2K/41.7K
> > I ran fio randread benchmarks with 4k, 16k, 64k, and 128k at iodepth 1,
> > 8, and 64 on two vCPUs. The results look fine, there is no significant
> > regression.
> > 
> > iodepth=1 and iodepth=64 are very consistent. For some reason the
> > iodepth=8 has significant variance but I don't think it's the fault of
> > this patch.
> > 
> > Fio results and the Jupyter notebook export are available here (check
> > out benchmark.html to see the graphs):
> > 
> > https://gitlab.com/stefanha/virt-playbooks/-/tree/virtio-blk-sgl-allocation-benchmark/notebook
> > 
> > Guest:
> > - Fedora 34
> > - Linux v5.14
> > - 2 vCPUs (pinned), 4 GB RAM (single host NUMA node)
> > - 1 IOThread (pinned)
> > - virtio-blk aio=native,cache=none,format=raw
> > - QEMU 6.1.0
> > 
> > Host:
> > - RHEL 8.3
> > - Linux 4.18.0-240.22.1.el8_3.x86_64
> > - Intel(R) Xeon(R) Silver 4214 CPU @ 2.20GHz
> > - Intel Optane DC P4800X
> > 
> > Stefan
> 
> Thanks, Stefan.
> 
> Would you like me to add some of the results in my commit msg ? or Tested-By
> sign ?

Thanks, there's no need to change the commit description.

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Tested-by: Stefan Hajnoczi <stefanha@redhat.com>

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Re: [PATCH v3 1/1] virtio-blk: avoid preallocating big SGL for data

[Michael S. Tsirkin]

OK by me.
Acked-by: Michael S. Tsirkin <mst@redhat.com>

I will queue it for the next kernel.
Thanks!


On Thu, Sep 23, 2021 at 04:40:56PM +0300, Max Gurtovoy wrote:
> Hi MST/Jens,
> 
> Do we need more review here or are we ok with the code and the test matrix ?
> 
> If we're ok, we need to decide if this goes through virtio PR or block PR.
> 
> Cheers,
> 
> -Max.
> 
> On 9/14/2021 3:22 PM, Stefan Hajnoczi wrote:
> > On Mon, Sep 13, 2021 at 05:50:21PM +0300, Max Gurtovoy wrote:
> > > On 9/6/2021 6:09 PM, Stefan Hajnoczi wrote:
> > > > On Wed, Sep 01, 2021 at 04:14:34PM +0300, Max Gurtovoy wrote:
> > > > > No need to pre-allocate a big buffer for the IO SGL anymore. If a device
> > > > > has lots of deep queues, preallocation for the sg list can consume
> > > > > substantial amounts of memory. For HW virtio-blk device, nr_hw_queues
> > > > > can be 64 or 128 and each queue's depth might be 128. This means the
> > > > > resulting preallocation for the data SGLs is big.
> > > > > 
> > > > > Switch to runtime allocation for SGL for lists longer than 2 entries.
> > > > > This is the approach used by NVMe drivers so it should be reasonable for
> > > > > virtio block as well. Runtime SGL allocation has always been the case
> > > > > for the legacy I/O path so this is nothing new.
> > > > > 
> > > > > The preallocated small SGL depends on SG_CHAIN so if the ARCH doesn't
> > > > > support SG_CHAIN, use only runtime allocation for the SGL.
> > > > > 
> > > > > Re-organize the setup of the IO request to fit the new sg chain
> > > > > mechanism.
> > > > > 
> > > > > No performance degradation was seen (fio libaio engine with 16 jobs and
> > > > > 128 iodepth):
> > > > > 
> > > > > IO size      IOPs Rand Read (before/after)         IOPs Rand Write (before/after)
> > > > > --------     ---------------------------------    ----------------------------------
> > > > > 512B          318K/316K                                    329K/325K
> > > > > 
> > > > > 4KB           323K/321K                                    353K/349K
> > > > > 
> > > > > 16KB          199K/208K                                    250K/275K
> > > > > 
> > > > > 128KB         36K/36.1K                                    39.2K/41.7K
> > > > I ran fio randread benchmarks with 4k, 16k, 64k, and 128k at iodepth 1,
> > > > 8, and 64 on two vCPUs. The results look fine, there is no significant
> > > > regression.
> > > > 
> > > > iodepth=1 and iodepth=64 are very consistent. For some reason the
> > > > iodepth=8 has significant variance but I don't think it's the fault of
> > > > this patch.
> > > > 
> > > > Fio results and the Jupyter notebook export are available here (check
> > > > out benchmark.html to see the graphs):
> > > > 
> > > > https://gitlab.com/stefanha/virt-playbooks/-/tree/virtio-blk-sgl-allocation-benchmark/notebook
> > > > 
> > > > Guest:
> > > > - Fedora 34
> > > > - Linux v5.14
> > > > - 2 vCPUs (pinned), 4 GB RAM (single host NUMA node)
> > > > - 1 IOThread (pinned)
> > > > - virtio-blk aio=native,cache=none,format=raw
> > > > - QEMU 6.1.0
> > > > 
> > > > Host:
> > > > - RHEL 8.3
> > > > - Linux 4.18.0-240.22.1.el8_3.x86_64
> > > > - Intel(R) Xeon(R) Silver 4214 CPU @ 2.20GHz
> > > > - Intel Optane DC P4800X
> > > > 
> > > > Stefan
> > > Thanks, Stefan.
> > > 
> > > Would you like me to add some of the results in my commit msg ? or Tested-By
> > > sign ?
> > Thanks, there's no need to change the commit description.
> > 
> > Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> > Tested-by: Stefan Hajnoczi <stefanha@redhat.com>

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Re: [PATCH v3 1/1] virtio-blk: avoid preallocating big SGL for data

[Michael S. Tsirkin]

My tree is ok.
Looks like your patch was developed on top of some other tree,
not plan upstream linux, so git am fails. I applied it using
patch and some manual tweaking, and it seems to work for
me but please do test it in linux-next and confirm -
will push to a linux-next branch in my tree soon.

On Thu, Sep 23, 2021 at 04:40:56PM +0300, Max Gurtovoy wrote:
> Hi MST/Jens,
> 
> Do we need more review here or are we ok with the code and the test matrix ?
> 
> If we're ok, we need to decide if this goes through virtio PR or block PR.
> 
> Cheers,
> 
> -Max.
> 
> On 9/14/2021 3:22 PM, Stefan Hajnoczi wrote:
> > On Mon, Sep 13, 2021 at 05:50:21PM +0300, Max Gurtovoy wrote:
> > > On 9/6/2021 6:09 PM, Stefan Hajnoczi wrote:
> > > > On Wed, Sep 01, 2021 at 04:14:34PM +0300, Max Gurtovoy wrote:
> > > > > No need to pre-allocate a big buffer for the IO SGL anymore. If a device
> > > > > has lots of deep queues, preallocation for the sg list can consume
> > > > > substantial amounts of memory. For HW virtio-blk device, nr_hw_queues
> > > > > can be 64 or 128 and each queue's depth might be 128. This means the
> > > > > resulting preallocation for the data SGLs is big.
> > > > > 
> > > > > Switch to runtime allocation for SGL for lists longer than 2 entries.
> > > > > This is the approach used by NVMe drivers so it should be reasonable for
> > > > > virtio block as well. Runtime SGL allocation has always been the case
> > > > > for the legacy I/O path so this is nothing new.
> > > > > 
> > > > > The preallocated small SGL depends on SG_CHAIN so if the ARCH doesn't
> > > > > support SG_CHAIN, use only runtime allocation for the SGL.
> > > > > 
> > > > > Re-organize the setup of the IO request to fit the new sg chain
> > > > > mechanism.
> > > > > 
> > > > > No performance degradation was seen (fio libaio engine with 16 jobs and
> > > > > 128 iodepth):
> > > > > 
> > > > > IO size      IOPs Rand Read (before/after)         IOPs Rand Write (before/after)
> > > > > --------     ---------------------------------    ----------------------------------
> > > > > 512B          318K/316K                                    329K/325K
> > > > > 
> > > > > 4KB           323K/321K                                    353K/349K
> > > > > 
> > > > > 16KB          199K/208K                                    250K/275K
> > > > > 
> > > > > 128KB         36K/36.1K                                    39.2K/41.7K
> > > > I ran fio randread benchmarks with 4k, 16k, 64k, and 128k at iodepth 1,
> > > > 8, and 64 on two vCPUs. The results look fine, there is no significant
> > > > regression.
> > > > 
> > > > iodepth=1 and iodepth=64 are very consistent. For some reason the
> > > > iodepth=8 has significant variance but I don't think it's the fault of
> > > > this patch.
> > > > 
> > > > Fio results and the Jupyter notebook export are available here (check
> > > > out benchmark.html to see the graphs):
> > > > 
> > > > https://gitlab.com/stefanha/virt-playbooks/-/tree/virtio-blk-sgl-allocation-benchmark/notebook
> > > > 
> > > > Guest:
> > > > - Fedora 34
> > > > - Linux v5.14
> > > > - 2 vCPUs (pinned), 4 GB RAM (single host NUMA node)
> > > > - 1 IOThread (pinned)
> > > > - virtio-blk aio=native,cache=none,format=raw
> > > > - QEMU 6.1.0
> > > > 
> > > > Host:
> > > > - RHEL 8.3
> > > > - Linux 4.18.0-240.22.1.el8_3.x86_64
> > > > - Intel(R) Xeon(R) Silver 4214 CPU @ 2.20GHz
> > > > - Intel Optane DC P4800X
> > > > 
> > > > Stefan
> > > Thanks, Stefan.
> > > 
> > > Would you like me to add some of the results in my commit msg ? or Tested-By
> > > sign ?
> > Thanks, there's no need to change the commit description.
> > 
> > Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> > Tested-by: Stefan Hajnoczi <stefanha@redhat.com>

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Re: [PATCH v3 1/1] virtio-blk: avoid preallocating big SGL for data

[Christoph Hellwig]

Looks good,

Reviewed-by: Christoph Hellwig <hch@lst.de>
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