Re: [PATCH v8 0/3]: perf: reduce data loss when profiling highly parallel CPU bound workloads

From: Alexey Budankov
Date: Mon Sep 10 2018 - 06:40:23 EST

Hi Ingo,

On 10.09.2018 12:18, Ingo Molnar wrote:
> * Alexey Budankov <alexey.budankov@xxxxxxxxxxxxxxx> wrote:
>> Currently in record mode the tool implements trace writing serially.
>> The algorithm loops over mapped per-cpu data buffers and stores
>> ready data chunks into a trace file using write() system call.
>> At some circumstances the kernel may lack free space in a buffer
>> because the other buffer's half is not yet written to disk due to
>> some other buffer's data writing by the tool at the moment.
>> Thus serial trace writing implementation may cause the kernel
>> to loose profiling data and that is what observed when profiling
>> highly parallel CPU bound workloads on machines with big number
>> of cores.
> Yay! I saw this frequently on a 120-CPU box (hw is broken now).
>> Data loss metrics is the ratio lost_time/elapsed_time where
>> lost_time is the sum of time intervals containing PERF_RECORD_LOST
>> records and elapsed_time is the elapsed application run time
>> under profiling.
>> Applying asynchronous trace streaming thru Posix AIO API
>> (
>> lowers data loss metrics value providing 2x improvement -
>> lowering 98% loss to almost 0%.
> Hm, instead of AIO why don't we use explicit threads instead? I think Posix AIO will fall back
> to threads anyway when there's no kernel AIO support (which there probably isn't for perf
> events).

Explicit threading is surely an option but having more threads
in the tool that stream performance data is a considerable
design complication.

Luckily, glibc AIO implementation is already based on pthreads,
but having a writing thread for every distinct fd only.

> Per-CPU threading the record session would have so many other advantages as well (scalability,
> etc.).>
> Jiri did per-CPU recording patches a couple of months ago, not sure how usable they are at the
> moment?

Tool threads may contend, and actually do, with application
threads, under heavy load when all CPU cores are utilized,
and this may alter performance profile.


> Thanks,
> Ingo