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

From: Stefan Hajnoczi <stefanha@redhat.com>
To: Max Gurtovoy <mgurtovoy@nvidia.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 13:21:27 +0100	[thread overview]
Message-ID: <YTDBx/E/UJZWTFlG@stefanha-x1.localdomain> (raw)
In-Reply-To: <20210901131434.31158-1-mgurtovoy@nvidia.com>

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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?

Stefan

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