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

[Jiri Olsa]

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>

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/