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

[Peter Zijlstra]

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..).

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,