Re: [PATCH] kswapd shall not sleep during page shortage

[Marcelo Tosatti <marcelo.tosatti@cyclades.com>]

On Tue, Nov 09, 2004 at 04:28:01PM -0800, Andrew Morton wrote:
> Marcelo Tosatti <marcelo.tosatti@cyclades.com> wrote:
> >
> > Back to arguing in favour of my patch - it seemed to me that kswapd could 
> >  go to sleep leaving allocators which can't reclaim pages themselves in a 
> >  bad situation. 
> 
> Yes, but those processes would be sleeping in blk_congestion_wait() during,
> say, a GFP_NOIO/GFP_NOFS allocation attempt. 

I was thinking about interrupts when I mentioned "allocators which can't reclaim 
pages" :)

> And in that case, they may be
> holding locks whcih prevent kswapd from being able to do any work either.

OK... Just out of curiosity:
Isnt the "lock contention" at this level (filesystem) a relatively rare situation? 

It could be a NFS lock for example? What other kind of lock?

> >  It would have to be waken up by another instance of alloc_pages to then 
> >  execute and start doing its job, while if it was executing already (madly 
> >  scanning as you say), the chance it would find freeable pages quite
> >  earlier.
> > 
> >  Note that not only disk IO can cause pages to become freeable. A user
> >  can give up its reference on pagecache page for example (leaving
> >  the page on LRU to be found and freed by kswapd).
> 
> yup.  Or munlock(), or direct-io completion. 
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