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Re: [Qemu-devel] [RFC PATCH 6/8] quorum: Avoid bdrv_aio_writev() for rew
Re: [Qemu-devel] [RFC PATCH 6/8] quorum: Avoid bdrv_aio_writev() for rewrites
Mon, 21 Nov 2016 12:56:36 +0100
Am 18.11.2016 um 22:11 hat Eric Blake geschrieben:
> On 11/18/2016 06:21 AM, Kevin Wolf wrote:
> >>> + ret = bdrv_co_pwritev(s->children[co->i],
> >>> + acb->sector_num * BDRV_SECTOR_SIZE,
> >>> + acb->nb_sectors * BDRV_SECTOR_SIZE,
> >>> + acb->qiov, 0);
> >>> + (void) ret;
> >> Why do you need 'ret' at all? If it's a placeholder to remind us to do
> >> something with this value in the future, you can simply add a FIXME
> >> comment.
> > I'm not sure whether we want to fix anything, it looks intentional to
> > me. I just wanted to be explicit about the ignored return value, both
> > for human readers and for tools like Coverity.
> In bdrv_co_flush(), we have:
> /* Return value is ignored - it's ok if wait queue is empty */
> I don't know if Coverity would squawk, but the cast to void looks a bit
> stranger to me than a comment, where what we did in bdrv_co_flush()
> seems reasonable. There's also the patch proposal to introduce
> ignore_value() in place of a cast to void, which is a bit more
> self-documenting about places that intentionally ignore a return value
> while still shutting Coverity up:
If you don't like the void cast, I can remove it. After all, the
bdrv_aio_writev() call in the original code didn't have it either. I'm
just surprised that it feels strange to both of you, I thought it was a
well-known idiom for "this value is intentionally unused".
> >>> + /* one less rewrite to do */
> >>> + acb->rewrite_count--;
> >>> + qemu_coroutine_enter_if_inactive(acb->co);
> >> I think you should only enter acb->co when acb->rewrite_count reaches
> >> zero.
> >> In all other cases the main coroutine simply iterates inside the while()
> >> loop, verifies that the number is still positive and yields again.
> >> The same applies to all other cases of qemu_coroutine_enter_if_inactive,
> >> by the way (I failed to notice it in patch #5).
> > I think I like it better this way because it keeps the loop condition
> > local to the caller instead of spreading it across the caller and the
> > places that reenter. On the other hand, I can see that not doing the
> > extra context switch might be a little more efficient.
> Do we have a feel for how many context switches this would save? If
> it's in the noise, cleaner code is probably a win; but if it is a
> hotspot, then we should definitely try the optimization.
Should normally be (num_children - 1) per request. With inconsistent
results and enabled rewrites, add another one for each child that needs
to be corrected.
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[Qemu-devel] [RFC PATCH 6/8] quorum: Avoid bdrv_aio_writev() for rewrites, Kevin Wolf, 2016/11/10
Re: [Qemu-devel] [RFC PATCH 0/8] quorum: Implement .bdrv_co_preadv/pwritev(), Paolo Bonzini, 2016/11/11
Re: [Qemu-devel] [RFC PATCH 0/8] quorum: Implement .bdrv_co_preadv/pwritev(), no-reply, 2016/11/12
- Re: [Qemu-devel] [RFC PATCH 7/8] quorum: Implement .bdrv_co_preadv/pwritev(), (continued)