Re: [PATCH]vmscan: add block plug for page reclaim

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 28 Jul 2011 09:04:20 +0800 Shaohua Li <shaohua.li@intel.com> wrote:

> > Using an additional 44 bytes of stack on that path is also
> > significant(ly bad).  But we need to fix that problem anyway.  One way
> > we could improve things in mm/vmscan.c is to move the blk_plug into
> > scan_control then get the scan_control off the stack in some manner. 
> > That's easy for kswapd: allocate one scan_control per kswapd at
> > startup.  Doing it for direct-reclaim would be a bit trickier...
> unfortunately, the direct-reclaim case is what cares about stack.
> 
> BTW, the scan_control can be dieted. may_unmap/may_swap/may_writepage
> can be a bit. swappiness < 100, so can be a char. order <= 11, can be a
> char. should I do it to cut the size?

All five will fit in a 32-bit word, at some expense in code size.

But I think first it would be better to work on a way of getting it all
off the stack, along with the blk_plug.

Could be done with a per-cpu array and CPU pinning, but CPU pinning is
a bit expensive nowadays.  Could put a scan_control* into the
tack_struct, but that's dopey.