[gentoo-dev] Optimizing performance

[Patrick Lauer <patrick@gentoo.org>]

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Hi all,

I was wondering if there are any sane ways to optimize the performance
of a Gentoo system.
Overoptimization (the well known "-O9 -fomgomg" CFLAGS etc.) tends to
make things unstable, which is of course not what we want. The "easy"
way out would be buying faster hardware, but that is usually not an
option ;-)

So ... what can be done to get the stable maximum out of your hardware?

In my experience (x86 centric - do other arches have different
"problems"?) the following is stable, but not necessarily the optimum:
- don't overtweak CFLAGS. "-O2 -march=$your_cpu_family" seems to be on
average the best, -O3 is often slower and can cause bugs
- don't do anything with ASFLAGS, LDFLAGS. This causes weird random
breakage (e.g. LDFLAGS="-O1" causes prelink to fail with "absurd"
errors) and doesn't give a noticeable performance boost
- check that all IDE disks use DMA mode, otherwise they are limited to
~16M/s with a huge CPU usage penalty. Sometimes (application-specific)
increasing the readahead with hdparm gives a huge throughput boost.
- kernel tweaks like preempt may increase the responsiveness of the
system, but often reduce throughput and may have unexpected sideeffects
like random audio stutter as well as random kernel crashes ;-)
- kernel tweaks like setting swappiness or using a different I/O
scheduler (CFQ, deadline) should help, but I'm not aware of any "real"
benchmarks except microbenchmarks (can create 1M files 10% faster!!!!! -
yes, but how does it behave with a normal workload?)
- using a "smarter" filesystem can dramatically improve performance at
the potential cost of reliability. As data on FS reliability is hard to
find from unbiased sources this becomes a religious issue ... migrating
from ext3 to reiserfs makes "emerge sync" extremely much faster, but is
reiserfs sustainable?

Are there any application-specific tweaks (e.g. "use the prefork MPM
with apache2")? What is known to break things, what has usually
beneficial behaviour? Are there any useful benchmarks that show the
performance difference between different settings?

Thanks for your input,

Patrick
-- 
Stand still, and let the rest of the universe move

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