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

[Alexey Budankov]

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@xxxxxxxxxx>

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/
>