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SubjectRe: [PATCH v14 0/3]: perf: reduce data loss when profiling highly parallel CPU bound workloads
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Hi,

On 15.10.2018 13:17, Jiri Olsa wrote:
> On Mon, Oct 15, 2018 at 09:26:09AM +0300, Alexey Budankov wrote:
>>
>> Currently in record mode the tool implements trace writing serially.
>> The algorithm loops over mapped per-cpu data buffers and stores
>> ready data chunks into a trace file using write() system call.
>>
>> At some circumstances the kernel may lack free space in a buffer
>> because the other buffer's half is not yet written to disk due to
>> some other buffer's data writing by the tool at the moment.
>>
>> Thus serial trace writing implementation may cause the kernel
>> to loose profiling data and that is what observed when profiling
>> highly parallel CPU bound workloads on machines with big number
>> of cores.
>>
>> Experiment with profiling matrix multiplication code executing 128
>> threads on Intel Xeon Phi (KNM) with 272 cores, like below,
>> demonstrates data loss metrics value of 98%:
>>
>> /usr/bin/time perf record -o /tmp/perf-ser.data -a -N -B -T -R -g \
>> --call-graph dwarf,1024 --user-regs=IP,SP,BP --switch-events \
>> -e cycles,instructions,ref-cycles,software/period=1,name=cs,config=0x3/Duk -- \
>> matrix.gcc
>
> I ran above on 24 cpu server and could not see the gain,
> but I guess I'd need much bigger server to see that
>
> anyway, the code is now nicely separated, and given the
> advertised results below I have no objections
>
> Reviewed-by: Jiri Olsa <jolsa@kernel.org>

Is the plan Jiri mentioned earlier to have it as a stand alone patch kit
or upstream the changes into mainline?

Thanks,
Alexey

>
> thanks,
> jirka
>
>
>>
>> Data loss metrics is the ratio lost_time/elapsed_time where
>> lost_time is the sum of time intervals containing PERF_RECORD_LOST
>> records and elapsed_time is the elapsed application run time
>> under profiling.
>>
>> Applying asynchronous trace streaming thru Posix AIO API [1] lowers
>> data loss metrics value providing 2x improvement (from 98% to ~1%)
>>
>> Asynchronous trace streaming is currently limited to glibc linkage.
>> musl libc [5] also provides Posix AIO API implementation, however
>> the patchkit is not tested with it. There may be other libc libraries
>> linked by Perf tool that currently lack Posix AIO API support [2],
>> [3], [4] so NO_AIO define may be used to limit Perf tool binary to
>> serial streaming only.
>>
>> ---
>> Alexey Budankov (3):
>> perf util: map data buffer for preserving collected data
>> perf record: enable asynchronous trace writing
>> perf record: extend trace writing to multi AIO
>>
>> tools/perf/Documentation/perf-record.txt | 5 +
>> tools/perf/Makefile.config | 5 +
>> tools/perf/Makefile.perf | 7 +-
>> tools/perf/builtin-record.c | 252 ++++++++++++++++++++++++++++++-
>> tools/perf/perf.h | 1 +
>> tools/perf/util/evlist.c | 6 +-
>> tools/perf/util/evlist.h | 2 +-
>> tools/perf/util/mmap.c | 146 +++++++++++++++++-
>> tools/perf/util/mmap.h | 26 +++-
>> 9 files changed, 439 insertions(+), 11 deletions(-)
>>
>> ---
>> Changes in v14:
>> - implement default nr_cblocks_default variable
>> - fix --aio option handling
>> Changes in v13:
>> - named new functions with _aio_ word
>> - grouped aio functions under single #ifdef HAVE_AIO_SUPPORT
>> - moved perf_mmap__aio_push() stub into header
>> - removed trailed white space
>> Changes in v12:
>> - applied stub functions design for the whole patch kit
>> - grouped AIO related data into a struct under struct perf_mmap
>> - implemented record__aio_get/set_pos(), record__aio_enabled()
>> - implemented simple --aio option
>> - extended --aio option to --aio-cblocks=<n>
>> Changes in v11:
>> - replacing the both lseek() syscalls in every loop iteration by the only
>> two syscalls just before and after the loop at record__mmap_read_evlist()
>> and advancing *in-flight* off file pos value at perf_mmap__aio_push()
>> Changes in v10:
>> - moved specific code to perf_mmap__aio_mmap(), perf_mmap__aio_munmap();
>> - adjusted error reporting by using %m
>> - avoided lseek() setting file pos back in case of record__aio_write() failure
>> - compacted code selecting between serial and AIO streaming
>> - optimized call places of record__mmap_read_sync()
>> - added description of aio-cblocks option into perf-record.txt
>> Changes in v9:
>> - enable AIO streaming only when --aio-cblocks option is specified explicitly
>> - enable AIO based implementation when linking with glibc only
>> - define NO_AIO to limit Perf binary to serial implementation
>> Changes in v8:
>> - run the whole thing thru checkpatch.pl and corrected found issues except
>> lines longer than 80 symbols
>> - corrected comments alignment and formatting
>> - moved multi AIO implementation into 3rd patch in the series
>> - implemented explicit cblocks array allocation
>> - split AIO completion check into separate record__aio_complete()
>> - set nr_cblocks default to 1 and max allowed value to 4
>> Changes in v7:
>> - implemented handling record.aio setting from perfconfig file
>> Changes in v6:
>> - adjusted setting of priorities for cblocks;
>> - handled errno == EAGAIN case from aio_write() return;
>> Changes in v5:
>> - resolved livelock on perf record -e intel_pt// -- dd if=/dev/zero of=/dev/null count=100000
>> - data loss metrics decreased from 25% to 2x in trialed configuration;
>> - reshaped layout of data structures;
>> - implemented --aio option;
>> - avoided nanosleep() prior calling aio_suspend();
>> - switched to per-cpu aio multi buffer record__aio_sync();
>> - record_mmap_read_sync() now does global sync just before
>> switching trace file or collection stop;
>> Changes in v4:
>> - converted mmap()/munmap() to malloc()/free() for mmap->data buffer management
>> - converted void *bf to struct perf_mmap *md in signatures
>> - written comment in perf_mmap__push() just before perf_mmap__get();
>> - written comment in record__mmap_read_sync() on possible restarting
>> of aio_write() operation and releasing perf_mmap object after all;
>> - added perf_mmap__put() for the cases of failed aio_write();
>> Changes in v3:
>> - written comments about nanosleep(0.5ms) call prior aio_suspend()
>> to cope with intrusiveness of its implementation in glibc;
>> - written comments about rationale behind coping profiling data
>> into mmap->data buffer;
>> Changes in v2:
>> - converted zalloc() to calloc() for allocation of mmap_aio array,
>> - cleared typo and adjusted fallback branch code;
>>
>> ---
>>
>> [1] http://man7.org/linux/man-pages/man7/aio.7.html
>> [2] https://android.googlesource.com/platform/bionic/+/master/docs/status.md
>> [3] https://www.uclibc.org/
>> [4] https://uclibc-ng.org/
>> [5] https://www.musl-libc.org/
>

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