Re: [resend][PATCH] mm: increase RECLAIM_DISTANCE to 30

[Balbir Singh]

* KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx> [2010-10-08 10:48:26]:

> Recently, Robert Mueller reported zone_reclaim_mode doesn't work
> properly on his new NUMA server (Dual Xeon E5520 + Intel S5520UR MB).
> He is using Cyrus IMAPd and it's built on a very traditional
> single-process model.
> 
>   * a master process which reads config files and manages the other
>     process
>   * multiple imapd processes, one per connection
>   * multiple pop3d processes, one per connection
>   * multiple lmtpd processes, one per connection
>   * periodical "cleanup" processes.
> 
> Then, there are thousands of independent processes. The problem is,
> recent Intel motherboard turn on zone_reclaim_mode by default and
> traditional prefork model software don't work fine on it.
> Unfortunatelly, Such model is still typical one even though 21th
> century. We can't ignore them.
> 
> This patch raise zone_reclaim_mode threshold to 30. 30 don't have
> specific meaning. but 20 mean one-hop QPI/Hypertransport and such
> relatively cheap 2-4 socket machine are often used for tradiotional
> server as above. The intention is, their machine don't use
> zone_reclaim_mode.
> 
> Note: ia64 and Power have arch specific RECLAIM_DISTANCE definition.
> then this patch doesn't change such high-end NUMA machine behavior.
> 
> Cc: Mel Gorman <mel@xxxxxxxxx>
> Cc: Bron Gondwana <brong@xxxxxxxxxxx>
> Cc: Robert Mueller <robm@xxxxxxxxxxx>
> Acked-by: Christoph Lameter <cl@xxxxxxxxx>
> Acked-by: David Rientjes <rientjes@xxxxxxxxxx>
> Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
> ---
>  include/linux/topology.h |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/include/linux/topology.h b/include/linux/topology.h
> index 64e084f..bfbec49 100644
> --- a/include/linux/topology.h
> +++ b/include/linux/topology.h
> @@ -60,7 +60,7 @@ int arch_update_cpu_topology(void);
>   * (in whatever arch specific measurement units returned by node_distance())
>   * then switch on zone reclaim on boot.
>   */
> -#define RECLAIM_DISTANCE 20
> +#define RECLAIM_DISTANCE 30
>  #endif
>  #ifndef PENALTY_FOR_NODE_WITH_CPUS
>  #define PENALTY_FOR_NODE_WITH_CPUS	(1)

I am not sure if this makes sense, since RECLAIM_DISTANCE is supposed
to be a hardware parameter. Could you please help clarify what the
access latency of a node with RECLAIM_DISTANCE 20 to that of a node
with RECLAIM_DISTANCE 30 is? Has the hardware definition of reclaim
distance changed?

I suspect the side effect is the zone_reclaim_mode is not set to 1 on
bootup for the 2-4 socket machines you mention, which results in
better VM behaviour?

-- 
	Three Cheers,
	Balbir
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