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Re: [Qemu-devel] Block I/O outside the QEMU global mutex was "Re: [RFC P
From: |
Anthony Liguori |
Subject: |
Re: [Qemu-devel] Block I/O outside the QEMU global mutex was "Re: [RFC PATCH 00/17] Support for multiple "AIO contexts"" |
Date: |
Wed, 10 Oct 2012 07:25:12 -0500 |
User-agent: |
Notmuch/0.13.2+93~ged93d79 (http://notmuchmail.org) Emacs/23.3.1 (x86_64-pc-linux-gnu) |
Paolo Bonzini <address@hidden> writes:
> Il 09/10/2012 20:26, Anthony Liguori ha scritto:
>> Paolo Bonzini <address@hidden> writes:
>>
>>> Il 09/10/2012 17:37, Anthony Liguori ha scritto:
>>>>>>>> In the very short term, I can imagine an aio fastpath that was only
>>>>>>>> implemented in terms of the device API. We could have a slow path that
>>>>>>>> acquired the BQL.
>>>>>>
>>>>>> Not sure I follow.
>>>>
>>>> As long as the ioeventfd thread can acquire qemu_mutex in order to call
>>>> bdrv_* functions. The new device-only API could do this under the
>>>> covers for everything but the linux-aio fast path initially.
>>>
>>> Ok, so it's about the locking. I'm not even sure we need locking if we
>>> have cooperative multitasking. For example if bdrv_aio_readv/writev
>>> is called from a VCPU thread, it can just schedule a bottom half for
>>> itself in the appropriate AioContext. Similarly for block jobs.
>>
>> Okay, let's separate out the two issues here though. One is whether we
>> need a device specific block API. The second is whether we should short
>> cut to a fast path in the short term and go after a fully unlocked bdrv_
>> layer in the long(shortish?) term.
>>
>> So let's talk about your proposal...
>>
>>> The only part where I'm not sure how it would work is bdrv_read/write,
>>> because of the strange "qemu_aio_wait() calls select with a lock taken".
>>> Maybe we can just forbid synchronous I/O if you set a non-default
>>> AioContext.
>>
>> Not sure how practical that is. The is an awful lot of sync I/O still left.
>
> Hmm, yeah, perhaps we need to bite the bullet and use a recursive lock.
> The lock would be taken by:
>
> - sync I/O ops
>
> - monitor commands that currently call bdrv_drain_all
>
> - aio_poll when calling bottom halves or handlers
>
> The rest of the proposal however would stand (especially with reference
> to block jobs).
>
> I think we can proceed incrementally. The first obvious step is to
> s/qemu_bh_new/aio_bh_new/ in the whole block layer (including the
> CoQueue stuff), which would also help fixing the qemu-char bug that Jan
> reported.
>
>>> This would be entirely hidden in the block layer. For example the
>>> following does it for bdrv_aio_readv/writev:
>>>
>>> diff --git a/block.c b/block.c
>>> index e95f613..7165e82 100644
>>> --- a/block.c
>>> +++ b/block.c
>>> @@ -3712,15 +3712,6 @@ static AIOPool bdrv_em_co_aio_pool = {
>>> .cancel = bdrv_aio_co_cancel_em,
>>> };
>>>
>>> -static void bdrv_co_em_bh(void *opaque)
>>> -{
>>> - BlockDriverAIOCBCoroutine *acb = opaque;
>>> -
>>> - acb->common.cb(acb->common.opaque, acb->req.error);
>>> - qemu_bh_delete(acb->bh);
>>> - qemu_aio_release(acb);
>>> -}
>>> -
>>> /* Invoke bdrv_co_do_readv/bdrv_co_do_writev */
>>> static void coroutine_fn bdrv_co_do_rw(void *opaque)
>>> {
>>> @@ -3735,8 +3726,17 @@ static void coroutine_fn bdrv_co_do_rw(void *opaque)
>>> acb->req.nb_sectors, acb->req.qiov, 0);
>>> }
>>>
>>> - acb->bh = qemu_bh_new(bdrv_co_em_bh, acb);
>>> - qemu_bh_schedule(acb->bh);
>>> + acb->common.cb(acb->common.opaque, acb->req.error);
>>> + qemu_aio_release(acb);
>>> +}
>>> +
>>> +static void bdrv_co_em_bh(void *opaque)
>>> +{
>>> + BlockDriverAIOCBCoroutine *acb = opaque;
>>> +
>>> + qemu_bh_delete(acb->bh);
>>> + co = qemu_coroutine_create(bdrv_co_do_rw);
>>> + qemu_coroutine_enter(co, acb);
>>> }
>>>
>>> static BlockDriverAIOCB *bdrv_co_aio_rw_vector(BlockDriverState *bs,
>>> @@ -3756,8 +3756,8 @@ static BlockDriverAIOCB
>>> *bdrv_co_aio_rw_vector(BlockDriverState *bs,
>>> acb->req.qiov = qiov;
>>> acb->is_write = is_write;
>>>
>>> - co = qemu_coroutine_create(bdrv_co_do_rw);
>>> - qemu_coroutine_enter(co, acb);
>>> + acb->bh = qemu_bh_new(bdrv_co_em_bh, acb);
>>> + qemu_bh_schedule(acb->bh);
>>>
>>> return &acb->common;
>>> }
>>>
>>>
>>> Then we can add a bdrv_aio_readv/writev_unlocked API to the protocols, which
>>> would run outside the bottom half and provide the desired fast path.
>>
>> This works for some of the block layer I think. How does this interact
>> with thread pools for AIO?
>>
>> But this wouldn't work well with things like NBD or curl, right? What's
>> the plan there?
>
> NBD uses coroutines; curl can use the non-unlocked
> bdrv_aio_readv/writev. In both cases they would execute in the
> dataplane thread. qcow2-over-raw would also execute its read/write code
> entirely from the dataplane thread, for example.
Does that mean that we'd stop processing the queue if we're waiting for
an I/O completion to handle meta data operations?
If that's the case, that probably will hurt performance overall.
Regards,
Anthony Liguori
>
> Paolo
- Re: [Qemu-devel] Block I/O outside the QEMU global mutex was "Re: [RFC PATCH 00/17] Support for multiple "AIO contexts"", (continued)
- Re: [Qemu-devel] Block I/O outside the QEMU global mutex was "Re: [RFC PATCH 00/17] Support for multiple "AIO contexts"", Stefan Hajnoczi, 2012/10/09
- Re: [Qemu-devel] Block I/O outside the QEMU global mutex was "Re: [RFC PATCH 00/17] Support for multiple "AIO contexts"", Anthony Liguori, 2012/10/09
- Re: [Qemu-devel] Block I/O outside the QEMU global mutex was "Re: [RFC PATCH 00/17] Support for multiple "AIO contexts"", Paolo Bonzini, 2012/10/09
- Re: [Qemu-devel] Block I/O outside the QEMU global mutex was "Re: [RFC PATCH 00/17] Support for multiple "AIO contexts"", Anthony Liguori, 2012/10/09
- Re: [Qemu-devel] Block I/O outside the QEMU global mutex was "Re: [RFC PATCH 00/17] Support for multiple "AIO contexts"", Paolo Bonzini, 2012/10/09
- Re: [Qemu-devel] Block I/O outside the QEMU global mutex was "Re: [RFC PATCH 00/17] Support for multiple "AIO contexts"", Anthony Liguori, 2012/10/09
- Re: [Qemu-devel] Block I/O outside the QEMU global mutex was "Re: [RFC PATCH 00/17] Support for multiple "AIO contexts"", Paolo Bonzini, 2012/10/10
- Re: [Qemu-devel] Block I/O outside the QEMU global mutex was "Re: [RFC PATCH 00/17] Support for multiple "AIO contexts"",
Anthony Liguori <=
- Re: [Qemu-devel] Block I/O outside the QEMU global mutex was "Re: [RFC PATCH 00/17] Support for multiple "AIO contexts"", Paolo Bonzini, 2012/10/10
- Re: [Qemu-devel] Block I/O outside the QEMU global mutex was "Re: [RFC PATCH 00/17] Support for multiple "AIO contexts"", Anthony Liguori, 2012/10/10
- Re: [Qemu-devel] Block I/O outside the QEMU global mutex was "Re: [RFC PATCH 00/17] Support for multiple "AIO contexts"", Kevin Wolf, 2012/10/11