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

From: Max Gurtovoy <mgurtovoy@nvidia.com>
To: Stefan Hajnoczi <stefanha@redhat.com>
Cc: <hch@infradead.org>, <mst@redhat.com>,
	<virtualization@lists.linux-foundation.org>,
	<kvm@vger.kernel.org>, <israelr@nvidia.com>, <nitzanc@nvidia.com>,
	<oren@nvidia.com>, <linux-block@vger.kernel.org>,
	<axboe@kernel.dk>
Subject: Re: [PATCH v3 1/1] virtio-blk: avoid preallocating big SGL for data
Date: Thu, 2 Sep 2021 15:41:02 +0300	[thread overview]
Message-ID: <c8107800-63fa-2ab5-235f-6f365e22abe1@nvidia.com> (raw)
In-Reply-To: <YTDBx/E/UJZWTFlG@stefanha-x1.localdomain>

On 9/2/2021 3:21 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
>>
>> 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?

Yes you can test v3.

Thanks,

Max.

>
> Stefan