[Devel] Re: [PATCH v2] memcg: reduce lock time at move charge (Was Re: [PATCH 04/10] memcg: disable local interrupts in lock_page_cgroup()

[Andrew Morton]

On Fri, 8 Oct 2010 13:37:12 +0900 KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com> wrote:

> On Thu, 7 Oct 2010 16:14:54 -0700
> Andrew Morton <akpm at linux-foundation.org> wrote:
> 
> > On Thu, 7 Oct 2010 17:04:05 +0900
> > KAMEZAWA Hiroyuki <kamezawa.hiroyu at jp.fujitsu.com> wrote:
> > 
> > > Now, at task migration among cgroup, memory cgroup scans page table and moving
> > > account if flags are properly set.
> > > 
> > > The core code, mem_cgroup_move_charge_pte_range() does
> > > 
> > >  	pte_offset_map_lock();
> > > 	for all ptes in a page table:
> > > 		1. look into page table, find_and_get a page
> > > 		2. remove it from LRU.
> > > 		3. move charge.
> > > 		4. putback to LRU. put_page()
> > > 	pte_offset_map_unlock();
> > > 
> > > for pte entries on a 3rd level? page table.
> > > 
> > > This pte_offset_map_lock seems a bit long. This patch modifies a rountine as
> > > 
> > > 	for 32 pages: pte_offset_map_lock()
> > > 		      find_and_get a page
> > > 		      record it
> > > 		      pte_offset_map_unlock()
> > > 	for all recorded pages
> > > 		      isolate it from LRU.
> > > 		      move charge
> > > 		      putback to LRU
> > > 	for all recorded pages
> > > 		      put_page()
> > 
> > The patch makes the code larger, more complex and slower!
> > 
> 
> Slower ?

Sure.  It walks the same data three times, potentially causing
thrashing in the L1 cache.  It takes and releases locks at a higher
frequency.  It increases the text size.

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