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

[Jens Axboe <axboe@kernel.dk>]

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