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From: | Paolo Bonzini |
Subject: | Re: [Qemu-devel] [PATCH for-2.1?!?] AioContext: speed up aio_notify |
Date: | Fri, 04 Jul 2014 08:51:18 +0200 |
User-agent: | Mozilla/5.0 (X11; Linux x86_64; rv:24.0) Gecko/20100101 Thunderbird/24.6.0 |
Il 03/07/2014 20:17, Stefan Hajnoczi ha scritto:
On Thu, Jul 3, 2014 at 6:59 PM, Paolo Bonzini <address@hidden> wrote: Looks like a very interesting optimization for the block layer. If we can get some performance results - especially how it improves the virtio-blk dataplane regression - maybe we should merge it for 2.1.@@ -247,9 +248,22 @@ ThreadPool *aio_get_thread_pool(AioContext *ctx) return ctx->thread_pool; } +void aio_set_dispatching(AioContext *ctx, bool dispatching) +{ + ctx->dispatching = dispatching; + /* Write ctx->dispatching before reading e.g. bh->scheduled. */ + if (!dispatching) { + smp_mb(); + }Is the if statement necessary? It seems like an optimization to avoid the memory barrier in 50% of the calls.
Yes.
But by avoiding the barrier it could cause other threads to "miss" the dispatching period on machines with weak memory models. I guess that's fine since we're not trying to optimize other threads but more focussing on callers from the event loop's own thread. Anyway, the if statement makes this function trickier than it has to be IMO. Want to drop it?
I think this is the last "trickiness" problem of this patch, but of course I have no problem removing it if the patch goes into 2.1. Ming Lei's latest message suggests it is not necessary though.
The patch should be tested at least on rbd or gluster, these are the main users of remote-thread notification via bottom halves.
Paolo
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