linux-kernel - Re: [PATCH v2] perf mem: improves DSO long names search speed with RB tree

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Date:	Wed, 17 Sep 2014 10:08:51 -0400
From:	Waiman Long <waiman.long@...com>
To:	Adrian Hunter <adrian.hunter@...el.com>
CC:	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
	Scott J Norton <scott.norton@...com>,
	Don Zickus <dzickus@...hat.com>, Jiri Olsa <jolsa@...nel.org>
Subject: Re: [PATCH v2] perf mem: improves DSO long names search speed with
 RB tree

On 09/17/2014 02:22 AM, Adrian Hunter wrote:
> On 09/16/2014 10:08 PM, Waiman Long wrote:
>> With workload that spawns and destroys many threads and processes,
>> it was found that perf-mem could took a long time to post-process
>> the perf data after the target workload had completed its operation.
>> The performance bottleneck was found to be searching and insertion
>> of the new DSO structures (thousands of them in this case).
>>
>> In a dual-socket Ivy-Bridge E7-4890 v2 machine (30-core, 60-thread),
>> the perf profile below shows what perf was doing after the profiled
>> AIM7 shared workload completed:
>>
>> -     83.94%  perf  libc-2.11.3.so     [.] __strcmp_sse42
>>     - __strcmp_sse42
>>        - 99.82% map__new
>>             machine__process_mmap_event
>>             perf_session_deliver_event
>>             perf_session__process_event
>>             __perf_session__process_events
>>             cmd_record
>>             cmd_mem
>>             run_builtin
>>             main
>>             __libc_start_main
>> -     13.17%  perf  perf               [.] __dsos__findnew
>>       __dsos__findnew
>>       map__new
>>       machine__process_mmap_event
>>       perf_session_deliver_event
>>       perf_session__process_event
>>       __perf_session__process_events
>>       cmd_record
>>       cmd_mem
>>       run_builtin
>>       main
>>       __libc_start_main
>>
>> So about 97% of CPU times were spent in the map__new() function
>> trying to insert new DSO entry into the DSO linked list. The whole
>> post-processing step took about 9 minutes.
>>
>> The DSO structures are currently searched linearly. So the total
>> processing time will be proportional to n^2.
>>
>> To overcome this performance problem, the DSO code is modified to
>> also put the DSO structures in a RB tree sorted by its long name
>> in additional to being in a simple linked list. With this change,
>> the processing time will become proportional to n*log(n) which will
>> be much quicker for large n. However, the short name will still
>> be searched using the old linear searching method which is slow.
>> With that patch in place, the same perf-mem post-processing step took
>> less than 30 seconds to complete.
>>
>> Signed-off-by: Waiman Long<Waiman.Long@...com>
>> ---
>>   tools/perf/util/dso.c |   87 ++++++++++++++++++++++++++++++++++++++++++++++--
>>   tools/perf/util/dso.h |    1 +
>>   2 files changed, 84 insertions(+), 4 deletions(-)
>>
>> diff --git a/tools/perf/util/dso.c b/tools/perf/util/dso.c
>> index 819f104..fccb2f0 100644
>> --- a/tools/perf/util/dso.c
>> +++ b/tools/perf/util/dso.c
>> @@ -611,17 +611,93 @@ struct dso *dso__kernel_findnew(struct machine *machine, const char *name,
>>   	return dso;
>>   }
>>
>> +/*
>> + * RB root of DSOs sorted by the long name
>> + */
>> +static struct rb_root dso__longname_root = { NULL };
> Global variable!
>
> Why not just change the lists to rbtrees i.e.

The DSOs can be searched either by its short name or its long name. The 
patch enables long name search with RB tree. The short name search is 
still using the linked list as the sort order will be different from the 
long name.

I think I can remove the linked list and do the short name search 
linearly by scanning from the leftmost to the rightmost in the RBtree. 
In that way, we can remove the redundant list head field in the DSO 
structure.

Cheers,
Longman

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