Re: [patch 00/20] VM pageout scalability improvements

[Rik van Riel <riel@redhat.com>]

On Sun, 23 Dec 2007 01:57:32 +0530
Balbir Singh <balbir@linux.vnet.ibm.com> wrote:
> Rik van Riel wrote:
> > On large memory systems, the VM can spend way too much time scanning
> > through pages that it cannot (or should not) evict from memory. Not
> > only does it use up CPU time, but it also provokes lock contention
> > and can leave large systems under memory presure in a catatonic state.
> 
> I remember you mentioning that by large memory systems you mean systems
> with at-least 128GB, does this definition still hold?

It depends on the workload.  Certain test cases can wedge the
VM with as little as 16GB of RAM.  Other workloads cause trouble
at 32 or 64GB, with the system sometimes hanging for several
minutes, all the CPUs in the pageout code and no actual swap IO.

On systems of 128GB and more, we have seen systems hang in the
pageout code overnight, without deciding what to swap out.
 
> > This patch series improves VM scalability by:
> > 
> > 1) making the locking a little more scalable
> > 
> > 2) putting filesystem backed, swap backed and non-reclaimable pages
> >    onto their own LRUs, so the system only scans the pages that it
> >    can/should evict from memory
> > 
> > 3) switching to SEQ replacement for the anonymous LRUs, so the
> >    number of pages that need to be scanned when the system
> >    starts swapping is bound to a reasonable number
> > 
> > The noreclaim patches come verbatim from Lee Schermerhorn and
> > Nick Piggin.  I have not taken a detailed look at them yet and
> > all I have done is fix the rejects against the latest -mm kernel.
> 
> Is there a consolidate patch available, it makes it easier to test.

I will make a big patch available with the next version.  I have
to upgrade my patch set to newer noreclaim patches from Lee and
add a few small cleanups elsewhere.

> > I am posting this series now because I would like to get more
> > feedback, while I am studying and improving the noreclaim patches
> > myself.
> 
> What kind of tests show the problem? I'll try and review and test the code.

The easiest test possible simply allocates a ton of memory and
then touches it all.  Enough memory that the system needs to go
into swap.

Once memory is full, you will see the VM scan like mad, with a
big CPU spike (clearing the referenced bits off all pages) before
it starts swapping out anything.  That big CPU spike should be
gone or greatly reduced with my patches.

On really huge systems, that big CPU spike can be enough for one
CPU to spend so much time in the VM that all the other CPUs join
it, and the system goes under in a big lock contention fest.

Besides, even single threadedly clearing the referenced bits on
1TB worth of memory can't result in acceptable latencies :)

In the real world, users with large JVMs on their servers, which
sometimes go a little into swap, can trigger this system.  All of
the CPUs end up scanning the active list, and all pages have the
referenced bit set.  Even if the system eventually recovers, it
might as well have been dead.

Going into swap a little should only take a little bit of time.
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