----- 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. Thanks, Mathieu > > 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 -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com _______________________________________________ lttng-dev mailing list [email protected] https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
