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

Bakul Shah bakul at
Fri Mar 24 21:38:32 UTC 2006

> > 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. šThe 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.
> Although it may hard for the kernel to guess, which pages it can reclaim 
> efficiently in the general case, my issuing of madvise with MADV_SEQUENTIONAL 
> should've given it a strong hint.

Yes, that is what I was saying.  If mmap read can be made as
efficient as the use of read() for this most common case,
there are benefits.  In effect we set up a fifo that rolls
along the mapped address range and the kernel processing and
the user processing are somewhat decoupled.

>                       Reading via mmap should never be slower, than via read 
> -- it should be just a notch faster, in fact...

Depends on the cost of mostly redundant processing of N
read() syscalls versus the cost of setting up and tearing
down multiple v2p mappings -- presumably page faults
can be avoided if the kernel fills in pages ahead of when
they are first accessed.  The cost of tlbmiss is likely
minor.  Probably the breakeven point is just a few read()

> I'm also quite certain, that fulfulling my "demands" would add quite a bit of 
> complexity to the mmap support in kernel, but hey, that's what the kernel is 
> there for :-)

An interesting thought experiment is to assume the system has
*no* read and write calls and see how far you can get with
the present mmap scheme and what extensions are needed to get
back the same functionality.  Yes, assume mmap & friends even
for serial IO!  I am betting that mmap can be simplified.
[Proof by handwaving elided; this screen is too small to fit
my hands :-)]

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