[rrd-developers] rrdcached: to mmap or not to mmap

[Thorsten von Eicken]

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