Reading via mmap stinks (Re: weird bugs with mmap-ing via NFS)

Mikhail Teterin mi+mx at
Thu Mar 23 19:04:00 UTC 2006

вівторок 21 березень 2006 17:48, Matthew Dillon Ви написали:
>     Reading via mmap() is very well optimized.

Actually, I can not agree here -- quite the opposite seems true. When running 
locally (no NFS involved) my compressor with the `-1' flag (fast, least 
effective compression), the program easily compresses faster, than it can 

The Opteron CPU is about 50% idle, *and so is the disk* producing only 15Mb/s. 
I guess, despite the noise I raised on this subject a year ago, reading via 
mmap continues to ignore the MADV_SEQUENTIONAL and has no other adaptability.

Unlike read, which uses buffering, mmap-reading still does not pre-fault the 
file's pieces in efficiently :-(

Although the program was written to compress files, that are _likely_ still in 
memory, when used with regular files, it exposes the lack of mmap 

This should be even more obvious, if you time searching for a string in a 
large file using grep vs. 'grep --mmap'.



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