Re: vma_merge issue

[Hugh Dickins <hugh.dickins@tiscali.co.uk>]

On Wed, 12 Aug 2009, William R Speirs wrote:
> Hugh Dickins wrote:
> > 
> > MADV_DONTNEED: brilliant idea, what a shame it doesn't work for you.
> > I'd been on the point of volunteering a bugfix to it to do what you
> > want, it would make sense; but there's a big but... we have sold
> > MADV_DONTNEED as an madvise that only needs non-exclusive access
> > to the mmap_sem, which means it can be used concurrently with faulting,
> > which has made it much more useful to glibc (I believe).  If we were
> > to fiddle with vmas and accounting and merging in there, it would go
> > back to needing exclusive mmap_sem, which would hurt important users.
> 
> For my own edification, hurt these users how? Performance? Serializing access
> during a MADV_DONTNEED? I wonder how big the "hurt" would be?

Performance, yes: serializing, yes.

I forget the details, others will have paid closer attention, I may
be making this up!  But it was something like garbage collection when
when freeing mallocs: it pays off if faults elsewhere in the address
space can occur concurrently, but bad news if exclusive mmap_sem
locks out those faults.  Big enough hurt to show up very badly in
some reallife multithreaded apps, and benchmarks hitting the issue.

> > A "refinement" to that suggestion is to put the file on tmpfs:
> > you will then get charged for RAM+swap as you use it, but you can
> > use madvise MADV_REMOVE to unmap pages, punching holes in the file,
> > freeing up those charges.  A little baroque, but I think it does
> > amount to a way of doing exactly what you wanted in the first place.
> 
> I like this (the refined) idea a lot. I coded it up and works as expected,
> and the way I initially want.
> 
> Thanks for taking the time and providing the solution... I appreciate it.

I'm very glad to hear that worked out: thanks for reporting back.

Hugh

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