first time user with out of memory question

timeless time at
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 
>> hits/sec.
> 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[1]. That's 40 get hits per CPU millisecond, so we 
should be able to sustain roughly 40,000 get hits/second[2][3][4].

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[5]. We can throw that consideration out.


[1] Combined user+system
[2] Assuming ratio of gets to sets doesn't change a lot.
[3] ...and Assuming memcached really is O(1) for every operation
[4] This is a 4-way SMP box, so the assumption that Memcached can have a 
full CPU to itself seems reasonable.
[5] The CPU usage at 800 get hits/sec is so low it's hard to correllate 
CPU load to work.

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