Messages in this thread | | | Date | Tue, 24 Mar 2015 10:18:34 -0600 | From | David Ahern <> | Subject | Re: [PATCH] perf record: Allow poll timeout to be specified |
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On 3/24/15 10:12 AM, Ingo Molnar wrote: > > * David Ahern <david.ahern@oracle.com> wrote: > >> Record currently wakes up based on watermarks to read events from >> the mmaps and write them out to the file. The result is a file that >> can have large blocks of events per mmap before a finished round >> event is added to the stream. This in turn affects the quantity of >> events that have to be passed through the ordered events queue >> before results can be displayed to the user. For commands like >> perf-script this can lead to long unnecessarily long delays before a >> user gets output. Large systems (e.g, 1024 cpus) further compound >> this effect. I have seen instances where I have to wait 45 minutes >> for perf-script to process a 5GB file before any events are shown. >> >> This patch adds an option to perf-record to allow a user to specify >> the poll timeout in msec. For example using 100 msec timeouts >> similar to perf-top means the mmaps are traversed much more >> frequently leading to a smoother analysis side. > > Please tune the default value (perhaps influenced by N_PROC?) so that > users will get sane behavior without having to specify this option!
I knew you were going to say that! ;-)
It's really a function of events coming in not cpus. The number of CPUs just compounds the problem.
I thought about making perf-record use a 100msec timeout like perf-top, but that can lead to unnecessary FINISHED_ROUND events in the file and unnecessary noise/overhead in the record side. On the other hand looking at scheduler tracepoints, kvm tracepoints, etc -- those can flood in to the point that even 100msec is too long.
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