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

[Max Gurtovoy <mgurtovoy@nvidia.com>]

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.


>
> 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


>
> 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 ?