mmap() question

[Konstantin Belousov]

On Sat, Oct 12, 2013 at 04:04:31PM +0400, Dmitry Sivachenko wrote:
> 
> On 12.10.2013, at 13:59, Konstantin Belousov <kostikbel at gmail.com> wrote:
> > 
> > I was not able to reproduce the situation locally. I even tried to start
> > a lot of threads accessing the mapped regions, to try to outrun the
> > pagedaemon. The user threads sleep on the disk read, while pagedaemon
> > has a lot of time to rebalance the queues. It might be a case when SSD
> > indeed makes a difference.
> > 
> 
> 
> With ordinary SATA drive it will take hours just to read 20GB of data from disk because of random access, it will do a lot of seeks and reading speed will be extremely low.
> 
> SSD dramatically improves reading speed.
> 
> 
> > Still, I see how this situation could appear. The code, which triggers
> > OOM, never fires if there is a free space in the swapfile, so the
> > absense of swap is neccessary condition to trigger the bug.  Next, OOM
> > calculation does not account for a possibility that almost all pages on
> > the queues can be reused. It just fires if free pages depleted too much
> > or free target cannot be reached.
> 
> 
> First I tried with some swap space configured.  The OS started to swap out my process after it reached about 20GB which is also not what I expected:  what is the reason to swap out regions of read-only mmap()ed files?  Is it the expected behaviour?
> 
How did you concluded that the pages from your r/o mappings were paged out ?
VM never does this.  Only anonymous memory could be written to swap file,
including the shadow pages for the writeable COW mappings.  I suspect that
you have another 20GB of something used on the machine meantime.

> 
> > 
> > Below is the prototype patch, against HEAD.  It is not applicable to
> > stable, please use HEAD kernel for test.
> 
> 
> Thanks, I will test the patch soon and report the results.
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