I've loved using %D microsecond request duration logging for helping track down
performance issues. But I've always thought, "wouldn't it be cool if you could
log how long Apache spent processing the request, versus how long it spent
*sending* the request?" So finally last week I said, "Self, why don't you
write it?" :)
I added a %w formatting directive to CustomLog via mod_log_config's
log_pfn_register, and I used request_rec's notes table to store the serverwait
microseconds until the request is finished. It's been a long time since I've
done much C, so my solution for storing the microsecond duration in the APR
"table" was:
static const int get_server_wait(request_rec *r) {
apr_time_t duration = apr_time_now() - r->request_time;
apr_table_setn(r->notes, "serverwait-usec",
apr_psprintf(r->pool, "%" APR_TIME_T_FMT,
apr_time_usec(duration)));
return OK;
}
I hooked this function into the Fix Ups phase, because it seemed the most
popular "end of apache processing phase." I still need to test in a proxy to
JBoss type situation, but I know at least using a simple CGI script it didn't
measure the CGI script processing, but I thought that might be unavoidable...
And we don't really have many CGI scripts in production anyway.
I've done some brief load testing with "ab" and so far I don't think it adds a
measurable amount of lag. I have two questions. First, might there be a
better hook phase to use to really capture Apache processing time? And second,
might there be a more efficient way to temporarily store the "serverwait-usec"
value than doing a string conversion?
I'd be happy to share the code if anyone's interested. Cheers!
- edan
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