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Re: [Qemu-devel] [PATCH 1/3] qsp: QEMU's Synchronization Profiler
From: |
Emilio G. Cota |
Subject: |
Re: [Qemu-devel] [PATCH 1/3] qsp: QEMU's Synchronization Profiler |
Date: |
Thu, 16 Aug 2018 00:29:33 -0400 |
User-agent: |
Mutt/1.9.4 (2018-02-28) |
On Wed, Aug 15, 2018 at 19:55:14 -0400, Emilio G. Cota wrote:
> On Wed, Aug 15, 2018 at 00:53:23 -0400, Emilio G. Cota wrote:
> > On Wed, Aug 15, 2018 at 11:09:42 +0800, Fam Zheng wrote:
> > > On Mon, 08/13 13:11, Emilio G. Cota wrote:
> > > > + --enable-sync-profiler) sync_profiler="yes"
> > > > + ;;
> > >
> > > Curious, not asking for a change: can this be made a runtime option
> > > instead of
> > > compile time, since there's no library dependencies? That should make this
> > > somewhat easier to use.
> >
> > Good point. I'll do some profiling tomorrow to see how the latency
> > of the locking primitives could be minimized (ideally, not using
> > the profiler should just add a well-predicted branch).
>
> I reduced it to just a branch, but still, I measured a few percentage
> points (1-2%, depending on the machine) slowdown when this is a
> run-time option. (This is for a bootup+shutdown test of a guest.)
>
> So I'll keep it as a build-time option, then.
I looked further into this. Turns out we don't need the branch at all;
we can make indirect calls via function pointers, where the
pointers are set by qsp_enable/disable().
Overhead of doing things this way is within noise range, since
compilers and CPUs are so good at dealing with indirect calls.
I'll send a v2 tomorrow.
Emilio
- Re: [Qemu-devel] [PATCH 2/3] monitor: show sync profiling info with 'info sync', (continued)
Re: [Qemu-devel] [PATCH 0/3] synchronization profiler, no-reply, 2018/08/15