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

From: Feng Li <lifeng1519@gmail.com>
To: Stefan Hajnoczi <stefanha@redhat.com>
Cc: Max Gurtovoy <mgurtovoy@nvidia.com>,
	linux-block <linux-block@vger.kernel.org>,
	kvm@vger.kernel.org, mst@redhat.com, israelr@nvidia.com,
	virtualization@lists.linux-foundation.org, hch@infradead.org,
	nitzanc@nvidia.com, Jens Axboe <axboe@kernel.dk>,
	oren@nvidia.com
Subject: Re: [PATCH v3 1/1] virtio-blk: avoid preallocating big SGL for data
Date: Fri, 10 Sep 2021 14:32:24 +0800	[thread overview]
Message-ID: <CAEK8JBAz8Y6b1a2v+_EhXowdSEQgpv0CxmYX1kMP+wN8W1qOdA@mail.gmail.com> (raw)
In-Reply-To: <YTYvOetMHvocg9UZ@stefanha-x1.localdomain>

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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