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

From: Ingo Molnar
Date: Mon Sep 10 2018 - 06:13:32 EST

* Jiri Olsa <jolsa@xxxxxxxxxx> wrote:

> On Mon, Sep 10, 2018 at 12:03:03PM +0200, Ingo Molnar wrote:
> >
> > * Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
> >
> > > > 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?
> > >
> > > it's still usable, I can rebase it and post a branch pointer,
> > > the problem is I haven't been able to find a case with a real
> > > performance benefit yet.. ;-)
> > >
> > > perhaps because I haven't tried on server with really big cpu
> > > numbers
> >
> > Maybe Alexey could pick up from there? Your concept looked fairly mature to me
> > and I tried it on a big-CPU box back then and there were real improvements.
> too bad u did not share your results, it could have been already in ;-)

Yeah :-/ Had a proper round of testing on my TODO, then the big box I'd have tested it on
broke ...

> let me rebase/repost once more and let's see


> I think we could benefit from both multiple threads event reading
> and AIO writing for it could be merged together

So instead of AIO writing, why not just turn into a directory structure
with per CPU files? That would allow all sorts of neat future performance features such as
mmap() or splice() based zero-copy.

User-space post-processing can then read the files and put them into global order - or use the
per CPU nature of them, which would be pretty useful too.

Also note how well this works on NUMA as well, as the backing pages would be allocated in a
NUMA-local fashion.

I.e. the whole per-CPU threading would enable such a separation of the tracing/event streams
and would allow true scalability.