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

From: Ingo Molnar
Date: Mon Sep 10 2018 - 08:06:51 EST

* Alexey Budankov <alexey.budankov@xxxxxxxxxxxxxxx> wrote:

> 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.

My argument is, we don't want to rely on glibc's choices here. They might
use a different threading design in the future, or it might differ between
libc versions.

The basic flow of tracing/profiling data is something we should control explicitly,
via explicit threading.

BTW., the usecase I was primarily concentrating on was a simpler one: 'perf record -a', not
inherited workflow tracing. For system-wide profiling the ideal tracing setup is clean per-CPU
separation, i.e. per CPU event fds, per CPU threads that read and then write into separate
per-CPU files.