Messages in this thread | | | Date | Mon, 04 Aug 2008 06:11:26 -0700 | From | Stephen Champion <> | Subject | Re: [Patch] Scale pidhash_shift/pidhash_size up based on num_possible_cpus(). |
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Eric W. Biederman wrote: > Robin Holt <holt@sgi.com> writes: >> Oops, confusing details. That was a different problem we had been >> tracking. > > Which leads back to the original question. What were you measuring > that showed improvement with a larger pid hash size? > > Almost by definition a larger hash table will perform better. However > my intuition is that we are talking about something that should be in > the noise for most workloads.
Robin asked me to chime in on this, as I did the early "look at that" work and suggested it to Robin.
I noticed the potential for increasing pid_shift while chasing down a patch to our kernel (2.6.16 stable based) which had proc_pid_readdir() calling find_pid() for init_task through the highest pid #. This patch caused a rather serious problem on a 2048 core Altix. Before identifying the culprit, I increased pidhash_shift. This made a *huge* difference: enough to get the box marginally functional while I tracked down the origins of the problem.
After backing out the problematic patch, I took a look at pidhash_shift in normal circumstances: With pidhash_shift == 12, running only a few common services and monitoring tools (sendmail, nagios, etc for ~28k active processes, mostly of the kernel variety), the 20 cpu boot cpuset we use on that system to confine normal system processes and interactive logins was spending >1% of it's time in find_pid(), and an 'ls /proc > /dev/null' took >0.4s. With pidhash_shift == 16, the timing went to <0.2, and the total time spent in find_pid() was reduced to noise level.
In addition to raising the limit on larger systems, it looked reasonable to scale the pid hash with the # processors instead of memory. While I observed variably high process:cpu ratios on small systems (2c - 32c), they also have relatively few processes. The 192c - 2048c systems I was able to look at were all hovering at 13 +/- 2 processes per cpu, even with wildly varying memory sizes.
Despite more recent changes in proc_pid_readdir, my results should apply to current source. It looks like both the old 2.6.16 implementation and the current version will call find_pid (or equivalent) once for each successive getdents() call on /proc, excepting when the cursor is on the first entry. A quick look, and we have 88 getdents64() calls both 'ps' and 'ls /proc' with 29k processes running, which appears to be the primary source of calls.
It's not giganormous, although I probably could come up with a pointless microbenchmark to show it's 300% better. Importantly, it does noticeably improve normal interactive tools like 'ps' and 'top', a performance visualization tool developed by a customer (nodemon) refreshes faster. For a 512k init allocation, that seems like a very good deal.
I'd like to lose 20,000 kernel processes in addition to growing the pid hash!
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