[lttng-dev] large liblttng-ust startup overhead (ust_lock)
Mathieu Desnoyers
mathieu.desnoyers at efficios.com
Tue Sep 5 14:51:42 UTC 2017
----- 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).
>
> 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 ?
Thanks,
Mathieu
>
> --
> 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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--
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com
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