Re: update re: fork() failures in 2.1.101

Re: update re: fork() failures in 2.1.101

[Rik van Riel]

[Paul get's "cannot fork" errors after 60 or more hours of
 uptime. This suggests fragmentation problems.]

On Thu, 11 Jun 1998, Paul Kimoto wrote:

> > Hmm, the 'cannot fork' issue only starting after some
> > days of uptime... This suggests fragmentation. Is your
> > box very heavily loaded, or just lightly (VM-wise)?
> 
> Light, I think; I have 48MB of RAM and usually end up with 8--16MB in swap.
> In normal operation I don't have to wait much for paging except for larger
> programs (netscape, xemacs, or big compilations).

Ahh, I think I see it now. The fragmentation on your system
persists because of the swap cache. The swap cache 'caches'
swap pages and kinda makes sure they are reloaded to the
same physical address.

Stephen, Ben: should we disable the swap cache when 
fragmentation is high?

Rik.
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Re: update re: fork() failures in 2.1.101

[Stephen C. Tweedie]

Hi,

On Fri, 12 Jun 1998 06:36:53 +0200 (MET DST), Rik van Riel
<H.H.vanRiel@phys.uu.nl> said:

> [Paul get's "cannot fork" errors after 60 or more hours of
>  uptime. This suggests fragmentation problems.]

Kernel version?

> Ahh, I think I see it now. The fragmentation on your system persists
> because of the swap cache. The swap cache 'caches' swap pages and
> kinda makes sure they are reloaded to the same physical address.

No.  As it stands right now, the "caching" component of the swap cache
is an *on disk* cache of resident pages.  Once the pages are swapped
out they are paged back in anywhere appropriate.  That part of the
fragmentation does not persist.

The real problem is not swapper, I suspect, but the various consumers of
slab cache (especially dcache).  The slab allocator has some really
nasty properties; just one single in-use object will pin an entire slab
(up to 32k) into memory.  If the slabs become small, then it will be 4k
pages which get so pinned, and at that point we cannot allocate any
stack pages.  There are a number of ways we may tackle this in 2.1, but
disabling the swap cache won't help at all.

--Stephen

Re: update re: fork() failures in 2.1.101

[Rik van Riel]

[CC-ed to linux-mm, and it should stay that way...]

On Thu, 18 Jun 1998, Paul Kimoto wrote:

> For completeness, here is the fragmentation report for each:
> > Jun 18 01:24:48   ( 48*4kB 7*8kB 1*16kB 1*32kB 4*64kB 1*128kB = 680kB)
> > Jun 18 18:03:53   ( 1*4kB 28*8kB 39*16kB 2*32kB 1*64kB 1*128kB = 1108kB)

Damn, this looks near-perfect for normal system load...
I really don't understand what's wrong.

> If you have other suggestions for things to try, with the reduction in
> memory (from 48 MB) the problems seem to arise in about half the time.

I wonder what kind of software / networking app you are using,
and what memory usage those programs have...

Rik.
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