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

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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