On Dienstag, 5. September 2017 20:11:58 CEST Mathieu Desnoyers wrote: > ----- On Sep 5, 2017, at 11:08 AM, Milian Wolff [email protected] wrote: > > On Tuesday, September 5, 2017 4:51:42 PM CEST Mathieu Desnoyers wrote: > >> ----- On Sep 5, 2017, at 10:34 AM, Milian Wolff [email protected] wrote: > >> > Hey all, > >> > > >> > I have noticed a very large overhead when linking against liblttng-ust: > >> > > >> > ~~~~~ > >> > ┌milian@milian-kdab2:/tmp > >> > └$ cat lttng-test.c > >> > int main() > >> > { > >> > > >> > return 0; > >> > > >> > } > >> > ┌milian@milian-kdab2:/tmp > >> > └$ gcc -O2 -g -ldl lttng-test.c > >> > ┌milian@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@milian-kdab2:/tmp > >> > └$ gcc -O2 -g -ldl -llttng-ust lttng-test.c > >> > ┌milian@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@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 > >> > $@ > >> > sudo lttng stop > >> > > >> > sudo chmod a+rx -R ~/lttng-traces > >> > ┌milian@milian-kdab2:/tmp > >> > └$ 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@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@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.
May I ask whether you have such a configuration available? The test case I have submitted should be easy to reproduce for everyone. Compiling my own kernel version takes a huge amount of time which I'm quite frankly not willing to invest at this point. I simply wonder whether anyone else is actually using lttng-ust, or if they are, whether they are only doing that for long-lived applications where this overhead is not noticeable? Thanks -- Milian Wolff | [email protected] | Senior Software Engineer KDAB (Deutschland) GmbH&Co KG, a KDAB Group company Tel: +49-30-521325470 KDAB - The Qt Experts _______________________________________________ lttng-dev mailing list [email protected] https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
