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

["Michael S. Tsirkin" <mst@redhat.com>]

OK by me.
Acked-by: Michael S. Tsirkin <mst@redhat.com>

I will queue it for the next kernel.
Thanks!


On Thu, Sep 23, 2021 at 04:40:56PM +0300, Max Gurtovoy wrote:
> Hi MST/Jens,
> 
> Do we need more review here or are we ok with the code and the test matrix ?
> 
> If we're ok, we need to decide if this goes through virtio PR or block PR.
> 
> Cheers,
> 
> -Max.
> 
> On 9/14/2021 3:22 PM, Stefan Hajnoczi wrote:
> > On Mon, Sep 13, 2021 at 05:50:21PM +0300, Max Gurtovoy wrote:
> > > 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 ?
> > Thanks, there's no need to change the commit description.
> > 
> > Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
> > Tested-by: Stefan Hajnoczi <stefanha@redhat.com>