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

[Roman Penyaev]

On Mon, May 30, 2016 at 8:40 AM, Alexandre DERUMIER <address@hidden> wrote:
> Hi,
>
>>>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:
>
> Could it be possible in the future to not open several times the same backend 
> ?

You are too fast :) I think nobody will do that in nearest future.

> I'm thinking about ceph/librbd, which since last version allow only to open 
> once a backend by default
> (exclusive-lock, which is a requirement for advanced features like 
> rbd-mirroring, fast-diff,....)

Consider my patch as a hack for only one reason: make true MQ support for
non-expandable file images and/or block devices to get some perf numbers
on lockless IO path.

If you are who is using block device as a backend and want to squeeze out
the IO till last drop from guest MQ bdev to host MQ bdev - feel free to
apply.  That's the only reason of this work.

--
Roman


>
> Regards,
>
> Alexandre Derumier
>
>
> ----- Mail original -----
> De: "Stefan Hajnoczi" <address@hidden>
> À: "Roman Pen" <address@hidden>
> Cc: "qemu-devel" <address@hidden>, "stefanha" <address@hidden>
> Envoyé: Samedi 28 Mai 2016 00:27:10
> Objet: Re: [Qemu-devel] [RFC] virtio-blk: simple multithreaded MQ 
> implementation for bdrv_raw
>
> 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