----- On Sep 6, 2017, at 3:57 PM, Mathieu Desnoyers [email protected] wrote:
> ----- On Sep 6, 2017, at 3:35 AM, Milian Wolff [email protected] wrote: > >> 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. > > We have reproduced the situation, and it's pointing towards our use of the > sys_membarrier system call. We'll try narrowing it down between its use by > lttng-ust (liburcu-bp.so) or by lttng-sessiond (liburcu.so). > > You can try to patch liburcu so it won't try using sys_membarrier at all > (see attached patch) > >> >> 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? > > It appears that this application startup slowdown mainly appears with > LTTng linked against liburcu 0.9 and 0.10 running with rather recent kernels. > This would explain why you are the first one to report this across our user > base. We have confirmed that it's the use of sys_membarrier SHARED by liburcu-bp, used by lttng-ust, that causes those delays. lttng-ust usage pattern is to call synchronize_rcu() directly. Having membarrier system calls each time adds up to significant delays, because membarrier SHARED blocks until a scheduler switch happens on each core. We are invoking synchronize_rcu() directly rather than using the call_rcu batching facilities because we want to minimize the number of threads we have to insert into the application. I'm currently proposing a PRIVATE_EXPEDITED command for the membarrier system call in Linux. It will actively send IPIs rather than blocking. I'm going to push a "Fix" in liburcu master, 0.10, 0.9 that will make it stop using sys_membarrier in liburcu-bp. Then, after the new PRIVATE_EXPEDITED command makes it way into the upstream Linux kernel, a future liburcu (0.11 or +) will introduce usage of that new command, given that it has a much, much smaller impact compared to the SHARED command. The reason why we want sys_membarrier there is to speed up tracing. When can trade memory barriers for simple compiler barriers on the tracing fast-path by using sys_membarrier on the update side. Thanks for reporting your finding! Mathieu > > 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 -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com _______________________________________________ lttng-dev mailing list [email protected] https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
