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
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