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

From: Peter Zijlstra <peterz@infradead.org>
To: Alexey Budankov <alexey.budankov@linux.intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>,
	Ingo Molnar <mingo@redhat.com>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Andi Kleen <ak@linux.intel.com>, Kan Liang <kan.liang@intel.com>,
	Dmitri Prokhorov <Dmitry.Prohorov@intel.com>,
	Valery Cherepennikov <valery.cherepennikov@intel.com>,
	David Carrillo-Cisneros <davidcc@google.com>,
	Stephane Eranian <eranian@google.com>,
	Mark Rutland <mark.rutland@arm.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH]: perf/core: addressing 4x slowdown during per-process profiling of STREAM benchmark on Intel Xeon Phi
Date: Mon, 29 May 2017 17:29:31 +0200	[thread overview]
Message-ID: <20170529152931.yxapmev3ix3hn5bk@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <20170529152254.wjx3b6apmatcso77@hirez.programming.kicks-ass.net>

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@linux.intel.com> 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,
> 

Oh and in general multi-PMU stuff, aside from hetero PMU becomes much
easier.