Re: + drop-pagecache.patch added to -mm tree

[Andrew Morton <akpm@osdl.org>]

Christoph Hellwig <hch@infradead.org> wrote:
>
> On Thu, Dec 29, 2005 at 05:54:08PM +0800, Coywolf Qi Hunt wrote:
> > 2005/12/2, akpm@osdl.org <akpm@osdl.org>:
> > >
> > > The patch titled
> > >
> > >      drop-pagecache
> > >
> > > has been added to the -mm tree.  Its filename is
> > >
> > >      drop-pagecache.patch
> > >
> > >
> > > From: Andrew Morton <akpm@osdl.org>
> > >
> > > Add /proc/sys/vm/drop-pagecache.  When written to, this will cause the kernel
> > > to discard as much pagecache and reclaimable slab objects as it can.
> > >
> > > It won't drop dirty data, so the user should run `sync' first.
> > >
> > > Caveats:
> > >
> > > a) Holds inode_lock for exorbitant amounts of time.
> > >
> > > b) Needs to be taught about NUMA nodes: propagate these all the way through
> > >    so the discarding can be controlled on a per-node basis.
> > >
> > > c) The pagecache shrinking and slab shrinking should probably have separate
> > >    controls.
> 
> d) it is a total mess.

Rubbish.

> A lot of code

295 bytes.

> for something that you shouldn't do
> except for benchmarking.

Sure.  It's a debugging feature.

>  If people see problems where pagecache data isn't
> dropped enough we should fix the VM instead of adding code that just bloats
> the kernel more.

It's useful for debugging VM problems, too.


I'm not fussed, really - it's useful for kernel developers and testers.  If
we're so worried about a very small amount of not-at-all-messy code then we
can stick it under CONFIG_DEBUG_VM.