Re: [PATCH 00/31] Move LRU page reclaim from zones to nodes v8

[Minchan Kim]

On Mon, Jul 04, 2016 at 10:55:09AM +0100, Mel Gorman wrote:
> On Mon, Jul 04, 2016 at 05:04:12PM +0900, Minchan Kim wrote:
> > > > How big ratio between highmem:lowmem do you think a problem?
> > > > 
> > > 
> > > That's a "how long is a piece of string" type question.  The ratio does
> > > not matter as much as whether the workload is both under memory pressure
> > > and requires large amounts of lowmem pages. Even on systems with very high
> > > ratios, it may not be a problem if HIGHPTE is enabled.
> > 
> > As well page table, pgd/kernelstack/zbud/slab and so on, every kernel
> > allocations wanted to mask __GFP_HIGHMEM off would be a problem in
> > 32bit system.
> > 
> 
> The same point applies -- it depends on the rate of these allocations,
> not the ratio of highmem:lowmem per se.
> 
> > It also depends on that how many drivers needed lowmem only we have
> > in the system.
> > 
> > I don't know how many such driver in the world. When I simply do grep,
> > I found several cases which mask __GFP_HIGHMEM off and among them,
> > I guess DRM might be a popular for us. However, it might be really rare
> > usecase among various i915 usecases.
> > 
> 
> It's also perfectly possible that such allocations are long-lived in which
> case they are not going to cause many skips. Hence why I cannot make a
> general prediction.
> 
> > > > > Conceptually, moving to node LRUs should be easier to understand. The
> > > > > page allocator plays fewer tricks to game reclaim and reclaim behaves
> > > > > similarly on all nodes. 
> > > > > 
> > > > > The series has been tested on a 16 core UMA machine and a 2-socket 48
> > > > > core NUMA machine. The UMA results are presented in most cases as the NUMA
> > > > > machine behaved similarly.
> > > > 
> > > > I guess you would already test below with various highmem system(e.g.,
> > > > 2:1, 3:1, 4:1 and so on). If you have, could you mind sharing it?
> > > > 
> > > 
> > > I haven't that data, the baseline distribution used doesn't even have
> > > 32-bit support. Even if it was, the results may not be that interesting.
> > > The workloads used were not necessarily going to trigger lowmem pressure
> > > as HIGHPTE was set on the 32-bit configs.
> > 
> > That means we didn't test this on 32-bit with highmem.
> > 
> 
> No. I tested the skip logic and noticed that when forced on purpose that
> system CPU usage was higher but it functionally worked.

Yeb, it would work well functionally. I meant not functionally but
performance point of view, system cpu usage and majfault rate
and so on.

> 
> > I'm not sure it's really too rare case to spend a time for testing.
> > In fact, I really want to test all series to our production system
> > which is 32bit and highmem but as we know well, most of embedded
> > system kernel is rather old so backporting needs lots of time and
> > care. However, if we miss testing in those system at the moment,
> > we will be suprised after 1~2 years.
> > 
> 
> It would be appreciated if it could be tested on such platforms if at all
> possible. Even if I did set up a 32-bit x86 system, it won't have the same
> allocation/reclaim profile as the platforms you are considering.

Yeb. I just finished reviewing of all patches and found no *big* problem
with my brain so my remanining homework is just testing which would find
what my brain have missed.

I will give the backporing to old 32-bit production kernel a shot and
report if something strange happens.

Thanks for great work, Mel!


> 
> > I don't know what kinds of benchmark can we can check it so I cannot
> > insist on it but you might know it.
> > 
> 
> One method would be to use fsmark with very large numbers of small files
> to force slab to require low memory. It's not representative of many real
> workloads unfortunately. Usually such a configuration is for checking the
> slab shrinker is working as expected.

Thanks for the suggestion.

> 
> > Okay, do you have any idea to fix it if we see such regression report
> > in 32-bit system in future?
> 
> Two options, neither whose complexity is justified without a "real"
> workload to use as a reference.
> 
> 1. Long-term isolation of highmem pages when reclaim is lowmem
> 
>    When pages are skipped, they are immediately added back onto the LRU
>    list. If lowmem reclaim persisted for long periods of time, the same
>    highmem pages get continually scanned. The idea would be that lowmem
>    keeps those pages on a separate list until a reclaim for highmem pages
>    arrives that splices the highmem pages back onto the LRU.
> 
>    That would reduce the skip rate, the potential corner case is that
>    highmem pages have to be scanned and reclaimed to free lowmem slab pages.
> 
> 2. Linear scan lowmem pages if the initial LRU shrink fails
> 
>    This will break LRU ordering but may be preferable and faster during
>    memory pressure than skipping LRU pages.

Okay. I guess it would be better to include this in descripion of [4/31].

> 
> -- 
> Mel Gorman
> SUSE Labs
> 
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