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Date: Sun, 20 Apr 2003 12:10:29 -0700
From: David Schultz <das@freebsd.org>
To: Matthew Dillon <dillon@apollo.backplane.com>
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cc: freebsd-current@freebsd.org
Subject: Re: Broken memory management on system with no swap
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On Sun, Apr 20, 2003, Matthew Dillon wrote:
>     Hmm.  It sounds like this program is using mmap() to dirty pages and 
>     that the VM system is not flushing them out quickly enough to avoid
>     running out of memory.  This could happen if the program dirties
>     a significant portion of memory all at once.  The pageout daemon would
>     wind up doing a priority requeue of the dirty pages (line 848 of
>     vm_pageout.c) and 'miss' flushing any of them out to the filesystem
>     in the first pass.  The result would be that the system would believe
>     it has run out of memory for a short period of time.

Thanks for your analysis.  I thought there might be a GBDE-related
factor when Lucky mentioned that copying a file triggers the bug,
since cp(1) turns off mmap() mode for files > 8 MB to avoid this
sort of thing.  But nevertheless, I can see how the situation you
describe can occur, where the system realizes too late that all of
the reclaimable pages are tied up in the active queue.

>     I suggest changing this:
> 
>         if ((vm_swap_size < 64 && vm_page_count_min()) ||
>             (swap_pager_full && vm_paging_target() > 0)) {
> 
>     To this:
> 
>         if (pass != 0 && 
> 	    ((vm_swap_size < 64 && vm_page_count_min()) ||
>             (swap_pager_full && vm_paging_target() > 0))) {

Sounds reasonable.  I would be happy to look into this next month
if nobody else is interested.  I would want to ensure that this
change cannot result in deadlock.