Re: all syscalls initially taking 4usec on a P4? Re: nonblockingUDPv4 recvfrom() taking 4usec @ 3GHz?

From: Rick Jones
Date: Tue Feb 20 2007 - 17:19:55 EST

I measure a huge slope, however. Starting at 1usec for back-to-back system
calls, it rises to 2usec after interleaving calls with a count to 20

4usec is hit after 110 million.

The graph, with semi-scientific error-bars is on

The code to generate it is on:

I'm investigating this further for other system calls. It might be that my
measurements are off, but it appears even a slight delay between calls
incurs a large penalty.

The slope appears to be flattening-out the farther out to the right it goes. Perhaps that is the length of time it takes to take all the requisite cache misses.

Some judicious use of HW perf counters might be in order via say papi or pfmon. Otherwise, you could try a test where you don't delay, but do try to blow-out the cache(s) between recvfrom() calls. If the delay there starts to match the delay as you go out to the right on the graph it would suggest that it is indeed cache effects.

rick jones
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