Re: [PATCH]: perf/core: addressing 4x slowdown during per-process profiling of STREAM benchmark on Intel Xeon Phi

[Alexey Budankov]

On 29.05.2017 18:29, Peter Zijlstra wrote:
> On Mon, May 29, 2017 at 05:22:54PM +0200, Peter Zijlstra wrote:
> > On Mon, May 29, 2017 at 04:43:09PM +0300, Alexey Budankov wrote:
> > > On 29.05.2017 15:03, Alexander Shishkin wrote:
> > > > Alexey Budankov <alexey.budankov@xxxxxxxxxxxxxxx> writes:
> >
> > > > > +		} else if (event->cpu > node_event->cpu) {
> > > > > +			node = &((*node)->rb_right);
> > > > > +		} else {
> > > > > +			list_add_tail(&event->group_list_entry,
> > > > > +					&node_event->group_list);
> > > >
> > > > So why is this better than simply having per-cpu event lists plus one
> > > > for per-thread events?
> > >
> > > Good question. Choice of data structure and layout depends on the operations
> > > applied to the data so keeping groups as a tree simplifies and improves the
> > > implementation in terms of scalability and performance. Please ask more if
> > > any.
> >
> > Since these lists are per context, and each task can have a context,
> > you'd end up with per-task-per-cpu memory, which is something we'd like
> > to avoid (some archs have very limited per-cpu memory space etc..).

Aw, yeah. Memory consumption does matter in the kernel space.

> > Also, we'd like to have that tree for other reasons, like for instance
> > that heterogeneous PMU crud ARM has. Also, with a tree we can easier do
> > time based round-robin scheduling,
>
> Oh and in general multi-PMU stuff, aside from hetero PMU becomes much
> easier.