I'm again trying to push the limit of rrdtool, specifically when using rrdcached. As long as the RRD working set fits in memory the performance is great. The fun begins when the RRD working set no longer fits in memory and disk reads start to be significant. One question I have is whether one should turn MMAP off (--disable-mmap) when operating servers in that regime in Linux. The rationale for turning it off is that rrdcached can be configured with multiple flush threads that can do concurrent read-modify-writes and, if I understand mmap and pthreads correctly, if I use mmap then one thread hitting a missing page blocks the entire process for the duration of the page fault, while without mmap one read hitting a missing block will only block that thread. Am I wrong? Cheers, Thorsten
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