Add GC.stat[:total_time] when GC profiling enabled
This patch includes a
:total_time value in
GC.stat as a
Fixnum of microseconds equivalent to
GC::Profiler.total_time. A non-zero value is only provided if GC profiling is enabled.
This avoids problems with
GC::Profiler's API in the presence of multiple callers. Well-behaved clients of
GC::Profiler are expected to call
clear periodically to constrain memory usage of the profiling structures. However, this causes problems when multiple callers--unaware of each other--rely on the value of
GC::Profiler.total_time being unaltered since their last
Using a single value in
GC.stat avoids this by providing a monotonically increasing count with every GC, not influenced by
Considerations and Questions
Because the individual GC times are potentially small, I tracked the total as a double and only convert to
GC.stat's return. Is there something better I should be doing there, in light of
GC.stat's insistence that the stats returned be if type
What should I do (if anything) to cope with potential overflows? If Ruby is on a platform where
size_tis 32 bits, we'd hit an overflow after 1.2 hours worth of cumulative GC time.
Is there any possibility of moving the GC timing (not the deeper profiling data) outside
GC::Profiler.enable's gating? I would love avoid clients needing to make code changes to track their GC time with my gem (newrelic_rpm).
As it stands invocation start times are held on the GC profiling structures, so it felt like a much bigger change to alter that, and I thought I'd pitch this simpler change first.
Any advice on whether that would be possible? Is timing too slow to do by default, or is the gating just an artifact of how it was implemented with
#1 [ruby-core:68461] Updated by ko1 (Koichi Sasada) over 2 years ago
With incremental GC (incremental marking and lazy sweep) consume very short time for each step.
I'm not sure we should measure such short time. At least, there are several BAD effect (especially on virtual machine environment).
If you want to measure only marking time, I can accept. But I'm not sure is it enough.
#2 [ruby-core:68467] Updated by jasonrclark (Jason Clark) over 2 years ago
That makes sense, Koichi. I wondered if timing the small steps by default would be too heavy.
By only measuring mark time, do you mean https://github.com/ruby/ruby/blob/trunk/gc.c#L5184-L5212? For my own curiosity, I'll probably see how those timings look. Even if they're not complete, they might be useful.
Also, I apologize for mixing a patch and a separate question into one issue. The patch doesn't actually add any timings--it's just a new
GC.stat counter, still gated by
GC::Profiler.enable. Does that seem reasonable as it avoids issues with multiple clients calling