----- On Jan 23, 2020, at 7:19 PM, lttng-dev lttng-dev@lists.lttng.org wrote:
> Hi, > > I recently installed knot dns for a very small FreeBSD server. I noticed > that it uses a surprising amount of CPU, even when there is no load: > about 0.25%. That's not huge, but it seems unnecessarily high when my > QPS is less than 0.01. > > After some profiling, I came to the conclusion that this is caused by > call_rcu_wait using futex_async to repeatedly wait. Since there is no > futex on FreeBSD (without the Linux compatibility layer), this > effectively turns into a permanent busy waiting loop. > > I think futex_noasync can be used here instead. call_rcu_wait is only > supposed to be called from call_rcu_thread, never from a signal context. > call_rcu calls get_call_rcu_data, which may call > get_default_call_rcu_data, which calls pthread_mutex_lock through > call_rcu_lock. Therefore, call_rcu is not async-signal-safe already. call_rcu() is meant to be async-signal-safe and lock-free after that initialization has been performed on first use. Paul, do you know where we have documented this in liburcu ? > Also, I think it only makes sense to use call_rcu around a RCU write, > which contradicts the README saying that only RCU reads are allowed in > signal handlers. Not sure what you mean by "use call_rcu around a RCU write" ? Is there anything similar to sys_futex on FreeBSD ? It would be good to look into alternative ways to fix this that do not involve changing the guarantees provided by call_rcu() for that fallback scenario (no futex available). Perhaps in your use-case you may want to tweak the retry delay for compat_futex_async(). Currently src/compat_futex.c:compat_futex_async() has a 10ms delay. Would 100ms be more acceptable ? Thanks, Mathieu > > I applied "sed -i -e 's/futex_async/futex_noasync/' > src/urcu-call-rcu-impl.h" and knot seems to work correctly with only > 0.01% CPU now. I also ran tests/unit and tests/regression with default > and signal backends and all completed successfully. > > I think that the other two usages of futex_async are also a little > suspicious, but I didn't look too closely. > > Thanks, > Alex. > _______________________________________________ > lttng-dev mailing list > lttng-dev@lists.lttng.org > https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com _______________________________________________ lttng-dev mailing list lttng-dev@lists.lttng.org https://lists.lttng.org/cgi-bin/mailman/listinfo/lttng-dev
