first time user with out of memory question
time at digg.com
Wed Jun 14 01:05:18 UTC 2006
>> Our memcached nodes utilise 1/50% CPU at 400 hits/sec. At volumes
>> around 400 hits/sec, the CPU:workload (C:W) ratio is <1. Assuming a
>> constant C:W ratio, this means we should utilise <<20% CPU at 400k
> By the way, if anyone actually cared to follow that paragraph, I
> recommend you discard what you might've gleaned from it. I've gone
> back to check my work and found a faulty assumption.
Here's the revised set of calculations.
During a short period of 800 get hits/second, we used 20,000 CPU
microseconds/second. That's 40 get hits per CPU millisecond, so we
should be able to sustain roughly 40,000 get hits/second.
One thing that does remain true from the aforequoted paragraph is that
our CPU per Work ratio is <1 (that is, doing double the work requires
less than double the CPU). Probably the difference is just the overhead
of running memcached. We can throw that consideration out.
 Combined user+system
 Assuming ratio of gets to sets doesn't change a lot.
 ...and Assuming memcached really is O(1) for every operation
 This is a 4-way SMP box, so the assumption that Memcached can have a
full CPU to itself seems reasonable.
 The CPU usage at 800 get hits/sec is so low it's hard to correllate
CPU load to work.
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