Re: [Qemu-devel] [RFC] virtio-blk: simple multithreaded MQ implementation for bdrv_raw

[Stefan Hajnoczi]

On Fri, May 27, 2016 at 01:55:04PM +0200, Roman Pen wrote:
> Hello, all.
> 
> This is RFC because mostly this patch is a quick attempt to get true
> multithreaded multiqueue support for a block device with native AIO.
> The goal is to squeeze everything possible on lockless IO path from
> MQ block on a guest to MQ block on a host.
> 
> To avoid any locks in qemu backend and not to introduce thread safety
> into qemu block-layer I open same backend device several times, one
> device per one MQ.  e.g. the following is the stack for a virtio-blk
> with num-queues=2:
> 
>             VirtIOBlock
>                /   \
>      VirtQueue#0   VirtQueue#1
>       IOThread#0    IOThread#1
>          BH#0          BH#1
>       Backend#0     Backend#1
>                \   /
>              /dev/null0
> 
> To group all objects related to one vq new structure is introduced:
> 
>     typedef struct VirtQueueCtx {
>         BlockBackend *blk;
>         struct VirtIOBlock *s;
>         VirtQueue *vq;
>         void *rq;
>         QEMUBH *bh;
>         QEMUBH *batch_notify_bh;
>         IOThread *iothread;
>         Notifier insert_notifier;
>         Notifier remove_notifier;
>         /* Operation blocker on BDS */
>         Error *blocker;
>     } VirtQueueCtx;
> 
> And VirtIOBlock includes an array of these contexts:
> 
>      typedef struct VirtIOBlock {
>          VirtIODevice parent_obj;
>     +    VirtQueueCtx mq[VIRTIO_QUEUE_MAX];
>      ...
> 
> This patch is based on Stefan's series: "virtio-blk: multiqueue support",
> with minor difference: I reverted "virtio-blk: multiqueue batch notify",
> which does not make a lot sense when each VQ is handled by it's own
> iothread.
> 
> The qemu configuration stays the same, i.e. put num-queues=N and N
> iothreads will be started on demand and N drives will be opened:
> 
>     qemu -device virtio-blk-pci,num-queues=8
> 
> My configuration is the following:
> 
> host:
>     Intel(R) Core(TM) i7-4770 CPU @ 3.40GHz,
>     8 CPUs,
>     /dev/nullb0 as backend with the following parameters:
>       $ cat /sys/module/null_blk/parameters/submit_queues
>       8
>       $ cat /sys/module/null_blk/parameters/irqmode
>       1
> 
> guest:
>     8 VCPUs
> 
> qemu:
>     -object iothread,id=t0 \
>     -drive 
> if=none,id=d0,file=/dev/nullb0,format=raw,snapshot=off,cache=none,aio=native \
>     -device 
> virtio-blk-pci,num-queues=$N,iothread=t0,drive=d0,disable-modern=off,disable-legacy=on
> 
>     where $N varies during the tests.
> 
> fio:
>     [global]
>     description=Emulation of Storage Server Access Pattern
>     bssplit=512/20:1k/16:2k/9:4k/12:8k/19:16k/10:32k/8:64k/4
>     fadvise_hint=0
>     rw=randrw:2
>     direct=1
> 
>     ioengine=libaio
>     iodepth=64
>     iodepth_batch_submit=64
>     iodepth_batch_complete=64
>     numjobs=8
>     gtod_reduce=1
>     group_reporting=1
> 
>     time_based=1
>     runtime=30
> 
>     [job]
>     filename=/dev/vda
> 
> Results:
>     num-queues   RD bw      WR bw
>     ----------   -----      -----
> 
>     * with 1 iothread *
> 
>     1 thr 1 mq   1225MB/s   1221MB/s
>     1 thr 2 mq   1559MB/s   1553MB/s
>     1 thr 4 mq   1729MB/s   1725MB/s
>     1 thr 8 mq   1660MB/s   1655MB/s
> 
>     * with N iothreads *
> 
>     2 thr 2 mq   1845MB/s   1842MB/s
>     4 thr 4 mq   2187MB/s   2183MB/s
>     8 thr 8 mq   1383MB/s   1378MB/s
> 
> Obviously, 8 iothreads + 8 vcpu threads is too much for my machine
> with 8 CPUs, but 4 iothreads show quite good result.

Cool, thanks for trying this experiment and posting results.

It's encouraging to see the improvement.  Did you use any CPU affinity
settings to co-locate vcpu and iothreads onto host CPUs?

Stefan

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