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    Subject[PATCH v7 0/2]: perf: reduce data loss when profiling highly parallel CPU bound workloads
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    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

    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
    (http://man7.org/linux/man-pages/man7/aio.7.html)
    lowers data loss metrics value providing 2x improvement -
    lowering 98% loss to almost 0%.

    ---
    Alexey Budankov (2):
    perf util: map data buffer for preserving collected data
    perf record: enable asynchronous trace writing

    tools/perf/builtin-record.c | 197 +++++++++++++++++++++++++++++++++++++++++++-
    tools/perf/perf.h | 1 +
    tools/perf/util/evlist.c | 7 +-
    tools/perf/util/evlist.h | 3 +-
    tools/perf/util/mmap.c | 110 +++++++++++++++++++++----
    tools/perf/util/mmap.h | 10 ++-
    6 files changed, 302 insertions(+), 26 deletions(-)

    ---
    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;

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