Have found some potential performance or stability issues associated
with lots of mutex blocking in getting a parent span's context. Want
the ability to totally disable tracing to help debugging.
Explained in a comment:
We default these Max File/Map counts very high. This is basically a
backwards compatibility thing where we don't want to cause different
behavior for those who had previously set their system limits high,
and weren't experiencing any bad behavior. Ideally you want these set
a bit below your system limits.
set sane defaults. The performance overhead seems to be negligible, and this will allow us to obtain mutex and blocking profiles from running Pilosas by default.
close files after using them if global max is passed.
I originally implemented this without the global count—just always closing files
when done with them, and reopening for new writes. This was crazy slow for that
one test that uses mustSetBits in a big loop. I modified the test to use
importRoaring and everything worked better (though much more slowly).
After adding the global counter, I ran the tests with that one test using
mustSetBits again, and the performance was similar to master. After completing
this PR, I ran the tests with the max limit set to 5—they still passed but were
much slower.
these were occaisionally referenced elsewhere in the codebase - in all but one
case, there were workarounds that are actually better I think.
In the one case there wasn't I created a single top level DefaultConfig object
which is instantiated with all the default values and can be referred to if
necessary.
There was a bug in fragment.go with the way MaxWritesPerRequest was treated if
it was 0. Elsewhere, 0 meant no limit, but here, it would have caused a division
by 0.
Changed the default metrics provider from "nop" to "none", although "nop" will
still work. Previously, any value other than "statsd" or "expvar" was treated as
"nop", but I've changed this behavior to return an error if an invalid string is
provided. I think this is better behavior, because in the case that someone
bothered to change the default, they were probably interested in actually
getting stats, and might be annoyed when it silently failed.
