[lttng-dev] large liblttng-ust startup overhead (ust_lock)

Milian Wolff milian.wolff at kdab.com
Tue Sep 5 14:52:42 UTC 2017


On Tuesday, September 5, 2017 4:34:54 PM CEST Milian Wolff wrote:
> Hey all,
> 
> I have noticed a very large overhead when linking against liblttng-ust:
> 
> ~~~~~
>milian at milian-kdab2:/tmp
> └$ cat lttng-test.c
> int main()
> {
>   return 0;
> }
>milian at milian-kdab2:/tmp
> └$ gcc -O2 -g -ldl lttng-test.c
>milian at milian-kdab2:/tmp
> └$ perf stat -r 5 ./a.out
> 
>  Performance counter stats for './a.out' (5 runs):
> 
>           0.209587      task-clock (msec)         #    0.596 CPUs utilized
> ( +-  8.76% )
>                  0      context-switches          #    0.000 K/sec
>                  0      cpu-migrations            #    0.000 K/sec
>                 49      page-faults               #    0.235 M/sec
> ( +-  1.19% )
>            706,854      cycles                    #    3.373 GHz
> ( +-  8.82% )
>            773,603      instructions              #    1.09  insn per cycle
> ( +-  0.75% )
>            147,128      branches                  #  701.987 M/sec
> ( +-  0.70% )
>              4,096      branch-misses             #    2.78% of all branches
> ( +-  5.27% )
> 
>        0.000351422 seconds time elapsed
> ( +- 11.85% )
> 
>milian at milian-kdab2:/tmp
> └$ gcc -O2 -g -ldl -llttng-ust lttng-test.c
>milian at milian-kdab2:/tmp
> └$ perf stat -r 5 ./a.out
> 
>  Performance counter stats for './a.out' (5 runs):
> 
>           2.063040      task-clock (msec)         #    0.009 CPUs utilized
> ( +-  1.37% )
>                 44      context-switches          #    0.021 M/sec
> ( +-  1.95% )
>                  2      cpu-migrations            #    0.776 K/sec
> ( +- 25.00% )
>                209      page-faults               #    0.101 M/sec
> ( +-  0.34% )
>          7,053,686      cycles                    #    3.419 GHz
> ( +-  2.03% )
>          6,893,783      instructions              #    0.98  insn per cycle
> ( +-  0.25% )
>          1,342,492      branches                  #  650.735 M/sec
> ( +-  0.20% )
>             29,390      branch-misses             #    2.19% of all branches
> ( +-  0.61% )
> 
>        0.225597302 seconds time elapsed
> ( +-  6.68% )
> ~~~~~
> 
> This is without any LTTng session configured. If I enable LTTng kernel and
> userspace events, this becomes even worse:
> 
> ~~~~~
>milian at milian-kdab2:/tmp
> └$ cat $(which run_lttng_trace.sh)
> #!/bin/sh
> 
> if [ -z "$(pidof lttng-sessiond)" ]; then
>     sudo lttng-sessiond --daemonize
> fi
> 
> sudo lttng create -o ~/lttng-traces/$(date -Iseconds)
> sudo lttng enable-channel kernel -k --subbuf-size 16M --num-subbuf 8
> sudo lttng enable-event -c kernel -k -a
> sudo lttng enable-channel ust -u --subbuf-size 16M --num-subbuf 8
> sudo lttng enable-event -c ust -u lttng_ust_tracef:*
> sudo lttng start
> $@
> sudo lttng stop
> 
> sudo chmod a+rx -R ~/lttng-traces
>milian at milian-kdab2:/tmp
> └$ run_lttng_trace.sh perf stat -r 5 ./a.out
> Session auto-20170905-162818 created.
> Traces will be written in
> /home/milian/lttng-traces/2017-09-05T16:28:18+02:00 Kernel channel kernel
> enabled for session auto-20170905-162818
> All Kernel events are enabled in channel kernel
> UST channel ust enabled for session auto-20170905-162818
> UST event lttng_ust_tracef:* created in channel ust
> Tracing started for session auto-20170905-162818
> 
>  Performance counter stats for './a.out' (5 runs):
> 
>           4.937820      task-clock (msec)         #    0.005 CPUs utilized
> ( +-  1.28% )
>                117      context-switches          #    0.024 M/sec
> ( +-  0.70% )
>                  3      cpu-migrations            #    0.608 K/sec
> ( +- 23.57% )
>                245      page-faults               #    0.050 M/sec
> ( +-  0.20% )
>         16,541,355      cycles                    #    3.350 GHz
> ( +-  0.94% )
>         20,611,637      instructions              #    1.25  insn per cycle
> ( +-  0.23% )
>          3,580,525      branches                  #  725.123 M/sec
> ( +-  0.19% )
>             52,093      branch-misses             #    1.45% of all branches
> ( +-  1.73% )
> 
>        0.965545882 seconds time elapsed
> ( +- 16.96% )
> 
> Waiting for data availability...
> Tracing stopped for session auto-20170905-162818
> ~~~~~
> 
> I see this ~1s delay during startup in nearly all test applications where I
> wanted to use lttng-ust. In tracecompass, and according to perf sleep time
> profiling, this seems to be mostly due to extensive sleeping on mutex locks
> (from ust_lock in liblttng-ust). With perf trace, I also see membarrier
> taking a lot of time:
> 
> ~~~~~
> # without a running lttng session:
>milian at milian-kdab2:/tmp
> └$ perf trace --duration 1 ./a.out
>      6.492 (52.468 ms): a.out/23672 recvmsg(fd: 3<socket:[1178439]>, msg:
> 0x7fbe2fbb1070                  ) = 1
>      5.077 (54.271 ms): a.out/23671 futex(uaddr: 0x7fbe30d508a0, op:
> WAIT_BITSET|PRIV|CLKRT, utime: 0x7ffc474ff5a0, val3: 4294967295) = 0
>     59.598 (79.379 ms): a.out/23671 membarrier(cmd: 1
> ) = 0
>    138.984 (39.945 ms): a.out/23671 membarrier(cmd: 1
> ) = 0
>     59.621 (119.324 ms): a.out/23672 futex(uaddr: 0x7fbe303e54c0, op: WAIT|
> PRIV, val: 2                    ) = 0
>    179.045 (79.918 ms): a.out/23671 membarrier(cmd: 1
> ) = 0
>    258.971 (39.997 ms): a.out/23671 membarrier(cmd: 1
> ) = 0
>    299.052 (79.883 ms): a.out/23671 membarrier(cmd: 1
> ) = 0
>    378.942 (59.988 ms): a.out/23671 membarrier(cmd: 1
> ) = 0
>    439.022 (69.908 ms): a.out/23671 membarrier(cmd: 1
> ) = 0
>    508.937 (80.027 ms): a.out/23671 membarrier(cmd: 1
> ) = 0
> 
> # with a running lttng session:
>milian at milian-kdab2:/tmp
> └$ run_lttng_trace.sh perf trace --duration 1 ./a.out
>      6.666 (392.496 ms): a.out-ust/23790 recvmsg(fd: 3<socket:[1185362]>,
> msg: 0x7fcd81d8a0b0                  ) = 612
>    400.238 (68.452 ms): a.out-ust/23790 membarrier(cmd: 1
> ) = 0
>    468.698 (39.983 ms): a.out-ust/23790 membarrier(cmd: 1
> ) = 0
>    510.567 (98.113 ms): a.out-ust/23790 recvmsg(fd: 3<socket:[1185362]>,
> msg: 0x7fcd81d8a070                  ) = 1
>      6.467 (603.500 ms): a.out/23789 futex(uaddr: 0x7fcd82f298a0, op:
> WAIT_BITSET|PRIV|CLKRT, utime: 0x7fff4df6c970, val3: 4294967295) = 0
>    610.020 ( 6.307 ms): a.out/23789 futex(uaddr: 0x7fcd82f29960, op: WAIT|
> PRIV, val: 2                    ) = 0
>    616.446 (62.265 ms): a.out/23789 membarrier(cmd: 1
> ) = 0
>    678.721 (59.916 ms): a.out/23789 membarrier(cmd: 1
> ) = 0
>    616.332 (122.335 ms): a.out-ust/23790 futex(uaddr: 0x7fcd82f29960, op:
> WAIT|PRIV, val: 2                    ) = 0
>    738.976 (49.658 ms): a.out/23789 membarrier(cmd: 1
> ) = 0
>    788.642 (49.987 ms): a.out/23789 membarrier(cmd: 1
> ) = 0
>    739.014 (99.644 ms): a.out-ust/23790 futex(uaddr: 0x7fcd825be4c0, op:
> WAIT| PRIV, val: 2                    ) = 0
>    838.651 (49.977 ms): a.out/23789 membarrier(cmd: 1
> ) = 0
>    888.636 (69.983 ms): a.out/23789 membarrier(cmd: 1
> ) = 0
>    838.662 (119.978 ms): a.out-ust/23790 futex(uaddr: 0x7fcd825be4c0, op:
> WAIT|PRIV, val: 2                    ) = 0
>    958.739 (59.873 ms): a.out/23789 membarrier(cmd: 1
> ) = 0
>   1018.621 (80.058 ms): a.out/23789 membarrier(cmd: 1
> ) = 0
>   1098.782 (49.844 ms): a.out/23789 membarrier(cmd: 1
> ) = 0
>   1148.634 (59.998 ms): a.out/23789 membarrier(cmd: 1
> ) = 0
>   1208.731 (69.940 ms): a.out/23789 membarrier(cmd: 1
> ) = 0
>   1278.680 (79.947 ms): a.out/23789 membarrier(cmd: 1
> ) = 0
>   1358.727 (39.881 ms): a.out/23789 membarrier(cmd: 1
> ) = 0
>   1398.616 (39.980 ms): a.out/23789 membarrier(cmd: 1
> ) = 0
>      6.640 (1432.157 ms): a.out-ust/23791 futex(uaddr: 0x7fcd82f90000
> ) = -1 (null) INTERNAL ERROR: strerror_r(512, 0x7ffdbc5467a0, 128)=22
> ~~~~~
> 
> This makes lttng-ust pretty much unusable for any short-lived application.
> Is this a known limitation?
> 
> I'm using lttng-ust on Arch from the AUR packages, i.e. LTTng 2.9.4. I've
> seen this problem also on Ubuntu 17.04 though I didn't do any of the above
> measurements there. So far, I suspected the ust statedump to take too much
> time, but I disabled that one for the measurements above.
> 
> Any help would be appreciated

Here's the perf sleep record data when no session is configured:

~~~~~
100% = 848.063336375 ms

# Overhead      Pid:Command    Shared Object     Symbol          Source:Line
# ........  .................  ................  ..............  ...........
#
    76.50%    10336:a.out-ust  [kernel.vmlinux]  [k] __schedule  core.c:0
            |
            ---__schedule core.c:0
               schedule
               futex_wait_queue_me futex.c:0
               futex_wait futex.c:0
               do_futex
               sys_futex
               do_syscall_64
               return_from_SYSCALL_64 .tmp_entry_64.o:0
               syscall
               ust_listener_thread futex.h:65
               /usr/include/urcu/futex.h:65 (inline)
               /usr/include/urcu/futex.h:97 (inline)
               /home/milian/projects/src/lttng/lttng-ust/liblttng-ust/lttng-
ust-comm.c:1311 (inline)
               /home/milian/projects/src/lttng/lttng-ust/liblttng-ust/lttng-
ust-comm.c:1373 (inline)

    15.79%    10335:a.out-ust  [kernel.vmlinux]  [k] __schedule  core.c:0
            |
            ---__schedule core.c:0
               schedule
               |          
                --15.70%--futex_wait_queue_me futex.c:0
                          futex_wait futex.c:0
                          do_futex
                          sys_futex
                          do_syscall_64
                          return_from_SYSCALL_64 .tmp_entry_64.o:0
                          __lll_lock_wait
                          pthread_mutex_lock
                          |          
                           --15.69%--0x2338
                                     0x3b29
                                     __nptl_deallocate_tsd.part.4 
pthread_create.c:0
                                     start_thread pthread_create.c:0
                                     __clone

     7.72%    10334:a.out      [kernel.vmlinux]  [k] __schedule  core.c:0
            |
            ---__schedule core.c:0
               schedule
               futex_wait_queue_me futex.c:0
               futex_wait futex.c:0
               do_futex
               sys_futex
               do_syscall_64
               return_from_SYSCALL_64 .tmp_entry_64.o:0
               do_futex_wait sem_timedwait.c:0
               __new_sem_wait_slow sem_timedwait.c:0
               lttng_ust_init.part.3 lttng-ust-comm.c:1757
               call_init.part.0 dl-init.c:0
               _dl_init
               _dl_start_user

~~~~~

Here's the perf sleep record data for an existing session:

~~~~~
100% = 3.0793293265 s

# Overhead      Pid:Command    Shared Object     Symbol          Source:Line
# ........  .................  ................  ..............  ...........
#
    52.10%    10762:a.out-ust  [kernel.vmlinux]  [k] __schedule  core.c:0
            |
            ---__schedule core.c:0
               schedule
               futex_wait_queue_me futex.c:0
               futex_wait futex.c:0
               do_futex
               sys_futex
               do_syscall_64
               return_from_SYSCALL_64 .tmp_entry_64.o:0
               syscall
               ust_listener_thread futex.h:65
               /usr/include/urcu/futex.h:65 (inline)
               /usr/include/urcu/futex.h:97 (inline)
               /home/milian/projects/src/lttng/lttng-ust/liblttng-ust/lttng-
ust-comm.c:1311 (inline)
               /home/milian/projects/src/lttng/lttng-ust/liblttng-ust/lttng-
ust-comm.c:1373 (inline)

    27.09%    10761:a.out-ust  [kernel.vmlinux]  [k] __schedule  core.c:0
            |
            ---__schedule core.c:0
               schedule
               |          
               |--13.69%--schedule_timeout
               |          unix_stream_read_generic af_unix.c:0
               |          unix_stream_recvmsg af_unix.c:0
               |          sock_recvmsg
               |          ___sys_recvmsg socket.c:0
               |          __sys_recvmsg
               |          sys_recvmsg
               |          do_syscall_64
               |          return_from_SYSCALL_64 .tmp_entry_64.o:0
               |          __libc_recvmsg
               |          ustcomm_recv_unix_sock lttng-ust-comm.c:307
               |          
                --13.40%--futex_wait_queue_me futex.c:0
                          futex_wait futex.c:0
                          do_futex
                          sys_futex
                          do_syscall_64
                          return_from_SYSCALL_64 .tmp_entry_64.o:0
                          __lll_lock_wait
                          pthread_mutex_lock
                          |          
                          |--8.41%--0x2338
                          |          0x3b29
                          |          __nptl_deallocate_tsd.part.4 
pthread_create.c:0
                          |          start_thread pthread_create.c:0
                          |          __clone
                          |          
                           --4.99%--ust_lock_nocheck lttng-ust-comm.c:178

    20.81%    10760:a.out      [kernel.vmlinux]  [k] __schedule  core.c:0
            |
            ---__schedule core.c:0
               schedule
               |          
                --20.81%--futex_wait_queue_me futex.c:0
                          futex_wait futex.c:0
                          do_futex
                          sys_futex
                          do_syscall_64
                          return_from_SYSCALL_64 .tmp_entry_64.o:0
                          |          
                           --20.77%--do_futex_wait sem_timedwait.c:0
                                     __new_sem_wait_slow sem_timedwait.c:0
                                     lttng_ust_init.part.3 lttng-ust-comm.c:
1757
                                     call_init.part.0 dl-init.c:0
                                     _dl_init
                                     _dl_start_user

~~~~~

Hope that helps
-- 
Milian Wolff | milian.wolff at kdab.com | Senior Software Engineer
KDAB (Deutschland) GmbH&Co KG, a KDAB Group company
Tel: +49-30-521325470
KDAB - The Qt Experts
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