Messages in this thread | | | Date | Wed, 20 May 2020 23:52:34 +0200 | From | Jiri Olsa <> | Subject | Re: [PATCH] perf ordered_events: Optimise event object reuse |
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On Wed, May 20, 2020 at 02:00:49PM +0100, Matt Fleming wrote: > On Mon, 18 May, at 02:04:08PM, Jiri Olsa wrote: > > On Fri, May 15, 2020 at 10:01:51PM +0100, Matt Fleming wrote: > > > ordered_event objects can be placed on the free object cache list in any > > > order which means future allocations may not return objects at > > > sequential locations in memory. Getting non-contiguous objects from the > > > free cache has bad consequences when later iterating over those objects > > > in ordered_events__queue(). > > > > > > For example, large perf.data files can contain trillions of events and > > > since objects that are next to each other in the free cache linked list > > > can point to pretty much anywhere in the object address space, lots of > > > cycles in ordered_events__queue() are spent servicing DTLB misses. > > > > > > Implement the free object cache using the in-kernel implementation of > > > interval trees so that objects can always be allocated from the free > > > object cache in sequential order, improving spatial locality and > > > reducing DTLB misses. > > > > > > Here are some numbers showing the speed up (reducing in execution time) > > > when running perf sched latency on sched events data and perf report on > > > HW_CPU_CYCLES. > > > > really nice, few questions below > > > > > > > > $ perf stat --null -r 10 -- bash -c \ > > > "export PAGER=cat ; perf sched latency -i $file --stdio &>/dev/null" > > > > > > Nr events File Size Before After Speed up > > > -------------- --------- -------- ------- ---------- > > > 123318457470 29MB 0.2149 0.2440 -13.5% > > > > should we be concerned about small data and the extra processing? > > I didn't look into this slowdown originally because it's ~2.9 ms, but > FYI it looks like this is caused by: > > - Longer code paths (more instructions) > - More branches > - More branch mispredicts > > > maybe we could add some option that disables this, at leat to be > > able to compare times in the future > > Sure. Do you mean a command-line option or build-time config?
command line option would be great
SNIP
> > > diff --git a/tools/perf/tests/free-object-cache.c b/tools/perf/tests/free-object-cache.c > > > new file mode 100644 > > > index 000000000000..e4395ece7d2b > > > --- /dev/null > > > +++ b/tools/perf/tests/free-object-cache.c > > > @@ -0,0 +1,200 @@ > > > +// SPDX-License-Identifier: GPL-2.0 > > > +#include "tests.h" > > > +#include <linux/kernel.h> > > > + > > > +#define ordered_events__flush_time __test_ordered_events__flush_time > > > +#define ordered_events__first_time __test_ordered_events__first_time > > > +#define ordered_events__delete __test_ordered_events__delete > > > +#define ordered_events__init __test_ordered_events__init > > > +#define ordered_events__free __test_ordered_events__free > > > +#define ordered_events__queue __test_ordered_events__queue > > > +#define ordered_events__reinit __test_ordered_events__reinit > > > +#define ordered_events__flush __test_ordered_events__flush > > > > I'm excited to see these tests, but why is above needed? > > > > can't you use ordered-events interface as it is? you used only > > exported functions right? > > Nope, the tests in this file are unit tests so I'm testing > free_cache_{get,put} which are file-local functions by #include'ing > ordered-events.c. > > The above define are required to avoid duplicate symbol errors at > link-time, e.g. > > util/perf-in.o: In function `ordered_events__flush_time': > /home/matt/src/kernels/linux/tools/perf/util/ordered-events.c:461: multiple definition of `ordered_events__flush_time' > tests/perf-in.o:/home/matt/src/kernels/linux/tools/perf/tests/../util/ordered-events.c:461: first defined here > > There are other ways to resolve this (linker flags to change the > symbols) but I couldn't find any precedent with that, so this seemed > like the easiest and most obvious solution. I'm happy to fix this up any > other way if you have suggestions though.
hum, could we just make free_cache_{get,put} public?
thanks, jirka
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