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

Bakul Shah bakul at
Thu Mar 23 23:52:40 UTC 2006

> :	time fgrep meowmeowmeow /home/oh.0.dump
> :	2.167u 7.739s 1:25.21 11.6%     70+3701k 23663+0io 6pf+0w
> :	time fgrep --mmap  meowmeowmeow /home/oh.0.dump
> :	1.552u 7.109s 2:46.03 5.2%      18+1031k 156+0io 106327pf+0w
> :
> :Use a big enough file to bust the memory caching (oh.0.dump above is 2.9Gb),
> :I'm sure, you will have no problems reproducing this result.
>     106,000 page faults.  How many pages is a 2.9GB file?  If this is running
>     in 64-bit mode those would be 8K pages, right?  So that would come to 
>     around 380,000 pages.  About 1:4.  So, clearly the operating system 
>     *IS* pre-faulting multiple pages.  
>     In anycase, this sort of test is not really a good poster child for how
>     to use mmap().  Nobody in their right mind uses mmap() on datasets that
>     they expect to be uncacheable and which are accessed sequentially.  It's
>     just plain silly to use mmap() in that sort of circumstance. 

May be the OS needs "reclaim-behind" for the sequential case?
This way you can mmap many many pages and use a much smaller
pool of physical pages to back them.  The idea is for the VM
to reclaim pages N-k..N-1 when page N is accessed and allow
the same process to reuse this page.  Similar to read ahead,
where the OS schedules read of page N+k, N+k+1.. when page N
is accessed.  May be even use TCP algorithms to adjust the
backing buffer (window) size:-)

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