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

Mathieu Desnoyers mathieu.desnoyers at efficios.com
Tue Sep 5 18:11:58 UTC 2017


----- On Sep 5, 2017, at 11:08 AM, Milian Wolff milian.wolff at kdab.com wrote:

> On Tuesday, September 5, 2017 4:51:42 PM CEST Mathieu Desnoyers wrote:
>> ----- On Sep 5, 2017, at 10:34 AM, Milian Wolff milian.wolff at kdab.com 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?
>> 
>> This kind of delay is not completely unexpected, but a bit higher than I
>> would expect. I'm interested in figuring out if most of this delay happens
>> to be caused by sys_membarrier, either directly, or indirectly (through a
>> lock).
> 
> See my other email which adds some harder numbers from a perf sleep record.
> 
>> > 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
>> 
>> Can you reproduce with a Linux kernel that has CONFIG_MEMBARRIER=n and
>> share the performance results ?
> 
> I'd have to recompile the kernel, which is something I don't like to do. Why
> is this required, or put differently - why would the kernel option influence
> the userspace LTTng library?

liburcu 0.9.x and 0.10.x (a dependency of lttng-ust) uses the membarrier system
call to speed up tracing, at the expense of slower execution of tracing state
update.

I'm currently proposing a new sys_membarrier command for PRIVATE_EXPEDITED
(single-process) use cases which should execute much faster than the
non-expedited SHARED command currently implemented.

liburcu 0.9 and 0.10 detects that this system call is available, and uses
it when it can.

Thanks,

Mathieu


> 
> Thanks
> 
> --
> 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

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com


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