* Christoph Lameter <cl@xxxxxxxxx> wrote:
> On Mon, 12 Nov 2012, Peter Zijlstra wrote:
>
> > The biggest conceptual addition, beyond the elimination of
> > the home node, is that the scheduler is now able to
> > recognize 'private' versus 'shared' pages, by carefully
> > analyzing the pattern of how CPUs touch the working set
> > pages. The scheduler automatically recognizes tasks that
> > share memory with each other (and make dominant use of that
> > memory) - versus tasks that allocate and use their working
> > set privately.
>
> That is a key distinction to make and if this really works
> then that is major progress.
I posted updated benchmark results yesterday, and the approach
is indeed a performance breakthrough:
http://lkml.org/lkml/2012/11/12/330
It also made the code more generic and more maintainable from a
scheduler POV.
> > This new scheduler code is then able to group tasks that are
> > "memory related" via their memory access patterns together:
> > in the NUMA context moving them on the same node if
> > possible, and spreading them amongst nodes if they use
> > private memory.
>
> What happens if processes memory accesses are related but the
> common set of data does not fit into the memory provided by a
> single node?
The other (very common) node-overload case is that there are
more tasks for a shared piece of memory than fits on a single
node.
I have measured two such workloads, one is the Java SPEC
benchmark:
v3.7-vanilla: 494828 transactions/sec
v3.7-NUMA: 627228 transactions/sec [ +26.7% ]
the other is the 'numa01' testcase of autonumabench:
v3.7-vanilla: 340.3 seconds
v3.7-NUMA: 216.9 seconds [ +56% ]
> The correct resolution usually is in that case to interleasve
> the pages over both nodes in use.
I'd not go as far as to claim that to be a general rule: the
correct placement depends on the system and workload specifics:
how much memory is on each node, how many tasks run on each
node, and whether the access patterns and working set of the
tasks is symmetric amongst each other - which is not a given at
all.
Say consider a database server that executes small and large
queries over a large, memory-shared database, and has worker
tasks to clients, to serve each query. Depending on the nature
of the queries, interleaving can easily be the wrong thing to
do.
Thanks,
Ingo
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