Ravikiran G Thirumalai <[email protected]> wrote:
>
> On Mon, Jan 16, 2006 at 06:13:23PM -0800, Andrew Morton wrote:
> > Ravikiran G Thirumalai <[email protected]> wrote:
> > >
> > > Convert global dirty_exceeded flag to per-node node_dirty_exceeded.
> > >
> > > dirty_exceeded ping pongs between nodes in order to force all cpus in
> > > the system to increase the frequency of calls to balance_dirty_pages.
> > >
> > > Currently dirty_exceeded is used by balance_dirty_pages_ratelimited to
> > > force all CPUs in the system call balance_dirty_pages often, in order to
> > > reduce the amount of dirty pages in the entire system (based on
> > > dirty_thresh and one CPU exceeding thee ratelimits). As dirty_exceeded
> > > is a global variable, it will ping-pong between nodes of a NUMA system
> > > which is not good.
> >
> > Did you not test this obvious little optimisation?
>
> We ran the test we encountered this problem on with your patch.
> At first it looked like it did not help. But later we found that there was
> false sharing on this variable.
OK. That's a bit nasty, isn't it? It can work well or poorly for
different people depending upon vagaries of .config and the linker.
We should find out what it was sharing _with_. Could you please run
nm -n vmlinux| grep -C5 dirty_exceeded
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