https://bugzilla.wikimedia.org/show_bug.cgi?id=70177
Ori Livneh <o...@wikimedia.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Priority|Normal |High
CC| |aschulz4...@gmail.com
See Also| |https://bugzilla.wikimedia.
| |org/show_bug.cgi?id=68413
Assignee|wikibugs-l@lists.wikimedia. |tstarl...@wikimedia.org
|org |
Summary|Lua takes a lot more CPU |The way LuaSandbox measures
|time with HHVM jobrunners |CPU time is not thread-safe
--- Comment #1 from Ori Livneh <o...@wikimedia.org> ---
The issue is that LuaSandbox is not thread-safe. The figure you're seeing
conflates the CPU time of several concurrent threads of execution.
* LuaSandbox uses CLOCK_PROCESS_CPUTIME_ID to measure time, but
CLOCK_PROCESS_CPUTIME_ID measures CPU time consumed by all threads of the
calling process.
* LuaSandbox requests to be notified with a SIGEV_SIGNAL upon timer expiration,
but at any point in time, only only one such signal is queued per timer per
process.
* LuaSandbox uses the sigprocmask() system call to block signals, but its
behavior in a multithreaded process is unspecified. pthread_sigmask() must be
used instead.
* When the normal or emergency timers are stopped, any queued SIGEV_SIGNAL for
the entire process is flushed.
* When multiple threads are executing concurrently, the timer for the entire
process is repeatedly re-armed (i.e., the exiration is reset).
* Confusingly, LUASANDBOX_G(signal_handler_installed) is thread-local, not
global. This means every thread calls luasandbox_timer_install_handler. The
latter caches the old handler in another thread-local variable and reinstalls
it when LuaSandbox is deactivating. When LuaSandbox is active in multiple
threads, individual threads end up re-installing each other's signal handlers,
keeping each other alive.
The above applies specifically to the normal and emergency timers. The
profiling feature has its own bag of thread-safety / reentrancy issues, which I
propose we deal with in the context of bug 68413 rather than this bug.
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