On Mon, Sep 6, 2021 at 11:39 PM Stefan Hajnoczi <stefanha@xxxxxxxxxx> 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@xxxxxxxxx>
