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Re: [Qemu-devel] [PATCH 0/9] simplify usage of tracepoints, and connect
From: |
Paolo Bonzini |
Subject: |
Re: [Qemu-devel] [PATCH 0/9] simplify usage of tracepoints, and connect them to logging |
Date: |
Sun, 25 Oct 2015 15:39:54 +0100 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Thunderbird/38.3.0 |
On 25/10/2015 14:57, Peter Maydell wrote:
> > Opinions? I would like to have this in 2.5 if there is agreement.
>
> Have you done any performance testing to check that we don't have
> previously-nopped-out tracepoints in hot paths that now result in
> real code being generated?
There definitely are tracepoints in hot paths, depending on what your
workload here. Each I/O operation should trigger up to 3 tracepoints in
the block layer if I counted them right (thread_pool_submit,
thread_pool_complete, bdrv_aio_*), plus possibly some SCSI and virtqueue
tracepoints.
I haven't checked whether their cost (e.g. in terms of cache misses) has
an impact on performance. The obvious first optimization would be to
optimize the case of no tracepoints enabled at all, changing each if
(tracepoint_enabled[index]) to "unlikely(tracepoint_enabled_any) &&
tracepoint_enabled[index]".
All other patches of course are independent. I put 8 and 9 last because
they are a bit more controversial.
Paolo
- [Qemu-devel] [PATCH 2/9] trace: split trace_init_events out of trace_init_backends, (continued)
- [Qemu-devel] [PATCH 2/9] trace: split trace_init_events out of trace_init_backends, Paolo Bonzini, 2015/10/25
- [Qemu-devel] [PATCH 3/9] trace: split trace_init_file out of trace_init_backends, Paolo Bonzini, 2015/10/25
- [Qemu-devel] [PATCH 5/9] trace: add "-trace help", Paolo Bonzini, 2015/10/25
- [Qemu-devel] [PATCH 4/9] trace: add "-trace enable=...", Paolo Bonzini, 2015/10/25
- [Qemu-devel] [PATCH 6/9] log: do not unnecessarily include qom/cpu.h, Paolo Bonzini, 2015/10/25
- [Qemu-devel] [PATCH 7/9] trace: convert stderr backend to log, Paolo Bonzini, 2015/10/25
- [Qemu-devel] [PATCH 8/9] trace: switch default backend to "log", Paolo Bonzini, 2015/10/25
- [Qemu-devel] [PATCH 9/9] log: add "-d trace:PATTERN", Paolo Bonzini, 2015/10/25
- Re: [Qemu-devel] [PATCH 0/9] simplify usage of tracepoints, and connect them to logging, Peter Maydell, 2015/10/25
- Re: [Qemu-devel] [PATCH 0/9] simplify usage of tracepoints, and connect them to logging,
Paolo Bonzini <=
- Re: [Qemu-devel] [PATCH 0/9] simplify usage of tracepoints, and connect them to logging, Christian Borntraeger, 2015/10/25
- Re: [Qemu-devel] [PATCH 0/9] simplify usage of tracepoints, and connect them to logging, Christian Borntraeger, 2015/10/25