Re: [PATCH v4] zone_reclaim is always 0 by default

From: KOSAKI Motohiro
Date: Tue Jun 09 2009 - 09:49:26 EST


sorry for late responce. my e-mail reading speed is very slow ;-)

First, Could you please read past thread?
I think many topic of this mail are already discussed.

> On Thu, Jun 04, 2009 at 07:23:15PM +0900, KOSAKI Motohiro wrote:
> >
> > Current linux policy is, zone_reclaim_mode is enabled by default if the machine
> > has large remote node distance. it's because we could assume that large distance
> > mean large server until recently.
> >
> We don't make assumptions about the server being large, small or otherwise. The
> affinity tables reporting a distance of 20 or more is saying "remote memory
> has twice the latency of local memory". This is true irrespective of workload
> and implies that going off-node has a real penalty regardless of workload.

Now, we talk about off-node allocation vs unnecessary file cache dropping.
IOW, off-node allocation vs disk access.

Then, the worth doesn't only depend on off-node distance, but also depend on
workload IO tendency and IO speed.

Fujitsu has 64 core ia64 HPC box, zone-reclaim sometimes made performance
degression although its box.

So, I don't think this problem is small vs large machine issue.
nor i7 issue.
high-speed P2P CPU integrated memory controller expose old issue.

> > In general, workload depended configration shouldn't put into default settings.
> >
> > However, current code is long standing about two year. Highest POWER and IA64 HPC machine
> > (only) use this setting.
> >
> > Thus, x86 and almost rest architecture change default setting, but Only power and ia64
> > remain current configuration for backward-compatibility.
> >
> What about if it's x86-64-based NUMA but it's not i7 based. There, the
> NUMA distances might really mean something and that zone_reclaim behaviour
> is desirable.

I don't hope ignore AMD, I think it's common characterastic of P2P and
integrated memory controller machine.

Also, I don't hope detect CPU family or similar, because we need update
such code evey when Intel makes new cpu.

Can we detect P2P interconnect machine? I'm not sure.

> I think if we're going down the road of setting the default, it shouldn't be
> per-architecture defaults as such. Other choices for addressing this might be;
> 1. Make RECLAIM_DISTANCE a variable on x86. Set it to 20 by default, and 5
> (or some other sensible figure) on i7
> 2. There should be a per-arch modifier callback for the affinity
> distances. If the x86 code detects the CPU is an i7, it can reduce the
> reported latencies to be more in line with expected reality.
> 3. Do not use zone_reclaim() for file-backed data if more than 20% of memory
> overall is free. The difficulty is figuring out if the allocation is for
> file pages.
> 4. Change zone_reclaim_mode default to mean "do your best to figure it
> out". Patch 1 would default large distances to 1 to see what happens.
> Then apply a heuristic when in figure-it-out mode and using reclaim_mode == 1
> If we have locally reclaimed 2% of the nodes memory in file pages
> within the last 5 seconds when >= 20% of total physical memory was
> free, then set the reclaim_mode to 0 on the assumption the node is
> mostly caching pages and shouldn't be reclaimed to avoid excessive IO
> Option 1 would appear to be the most straight-forward but option 2
> should be doable. Option 3 and 4 could turn into a rats nest and I would
> consider those approaches a bit more drastic.

I think the key-point of option 1 and 2 are proper hardware detecting way.

option 3 and 4 are more prefere idea to me. I like workload adapted heuristic.
but you already pointed out its hard, because page-allocator don't know
allocation purpose ;)

> > @@ -10,6 +10,12 @@ struct device_node;
> >
> > #include <asm/mmzone.h>
> >
> > +/*
> > + * Distance above which we begin to use zone reclaim
> > + */
> > +#define RECLAIM_DISTANCE 20
> > +
> > +
> Where is the ia-64-specific modifier to RECAIM_DISTANCE?

arch/ia64/include/asm/topology.h has

* Distance above which we begin to use zone reclaim

I don't think distance==15 is machine independent proper definition.
but there is long lived definition ;)

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