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: Mon, 13 Sep 2021 17:50:21 +0300	[thread overview]
Message-ID: <692f8e81-8585-1d39-e7a4-576ae01438a1@nvidia.com> (raw)
In-Reply-To: <YTYvOetMHvocg9UZ@stefanha-x1.localdomain>

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 ?