Re: [PATCH 2/2] mm, numa: Migrate pages to local nodes quicker early in the lifetime of a task

From: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: Peter Zijlstra <peterz@infradead.org>,
	Ingo Molnar <mingo@kernel.org>, Jirka Hladky <jhladky@redhat.com>,
	Rik van Riel <riel@surriel.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>
Subject: Re: [PATCH 2/2] mm, numa: Migrate pages to local nodes quicker early in the lifetime of a task
Date: Wed, 3 Oct 2018 18:45:49 +0530	[thread overview]
Message-ID: <20181003131549.GB4488@linux.vnet.ibm.com> (raw)
In-Reply-To: <20181002182248.GB7003@techsingularity.net>

> > if we want to prioritize STREAM like workloads (i.e private faults) one simpler
> > fix could be to change the quadtraic equation
> > 
> > from:
> > 	if (!cpupid_pid_unset(last_cpupid) &&
> > 				cpupid_to_nid(last_cpupid) != dst_nid)
> > 		return false;
> > to:
> > 	if (!cpupid_pid_unset(last_cpupid) &&
> > 				cpupid_to_nid(last_cpupid) == dst_nid)
> > 		return true;
> > 
> > i.e to say if the group tasks likely consolidated to a node or the task was
> > moved to a different node but access were private, just move the memory.
> > 
> > The drawback though is we keep pulling memory everytime the task moves
> > across nodes. (which is probably restricted for long running tasks to some
> > extent by your fix)
> > 
> 
> This has way more consequences as it changes the behaviour for the entire
> lifetime of the workload. It could cause excessive migrations in the case
> where a machine is almost fully utilised and getting load balanced or in
> cases where tasks are pulled frequently cross-node (e.g. worker thread
> model or a pipelined computation).
> 
> I'm only looking to address the case where the load balancer spreads a
> workload early and the memory should move to the new node quickly. If it
> turns out there are cases where that decision is wrong, it gets remedied
> quickly but if your proposal is ever wrong, the system doesn't recover.
> 

Agree.

-- 
Thanks and Regards
Srikar Dronamraju