Re: how text page of executable are shared ?

[kanoj@google.engr.sgi.com (Kanoj Sarcar)]

> 
> Hi,
> 
> On Tue, Mar 28, 2000 at 10:58:00AM -0500, Mark Hahn wrote:
> > 
> > could you comment on a problem I'm seeing in the current (pre3) VM?
> > the situation is a 256M machine, otherwise idle (random daemons, no X,
> > couple ssh's) and a process that sequentially traverses 12 40M files
> > by mmaping them (and munmapping them, in order, one at a time.)
> > 
> > the observation is that all goes well until the ~6th file, when we 
> > run out of unused ram.  then we start _swapping_!  the point is that 
> > shrink_mmap should really be scavenging those now unmapped files,
> > shouldn't it?
> 
> Well, you've filled the whole of memory with recently referenced page 
> cache pages.  The page cache scanner can now scan the whole of physical
> memory without finding anything which is "old" enough to be evicted. 
> It is only natural that it will start swapping at that point!
> 
> The swapping should be brief if all is working properly, though, as the
> shrink_mmap() will rapidly find itself on the second pass over memory
> and will start finding things which have been aged on the first pass 
> and not used since.
> 
> --Stephen
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> 

As I mentioned to Mark in private mail, it might be worthwhile looking
into the possiblity of using MADV_DONTNEED to discard file pages from 
the cache. In some os'es, I think msync(MS_INVALIDATE) actually takes
the page out from the cache.

Kanoj
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