This is an archived copy of the Xen.org mailing list, which we have preserved to ensure that existing links to archives are not broken. The live archive, which contains the latest emails, can be found at http://lists.xen.org/
Home Products Support Community News


[Xen-devel] [RFC] Xen NUMA strategy

To: xen-devel@xxxxxxxxxxxxxxxxxxx
Subject: [Xen-devel] [RFC] Xen NUMA strategy
From: "Andre Przywara" <andre.przywara@xxxxxxx>
Date: Fri, 14 Sep 2007 14:05:26 +0200
Cc: "Xu, Anthony" <anthony.xu@xxxxxxxxx>
Delivery-date: Fri, 14 Sep 2007 05:07:27 -0700
Envelope-to: www-data@xxxxxxxxxxxxxxxxxx
List-help: <mailto:xen-devel-request@lists.xensource.com?subject=help>
List-id: Xen developer discussion <xen-devel.lists.xensource.com>
List-post: <mailto:xen-devel@lists.xensource.com>
List-subscribe: <http://lists.xensource.com/cgi-bin/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=subscribe>
List-unsubscribe: <http://lists.xensource.com/cgi-bin/mailman/listinfo/xen-devel>, <mailto:xen-devel-request@lists.xensource.com?subject=unsubscribe>
Sender: xen-devel-bounces@xxxxxxxxxxxxxxxxxxx
User-agent: Thunderbird (X11/20070409)

Anthony Xu and I have had some fruitful discussion about the further direction of the NUMA support in Xen, I wanted to share the results with the Xen community and start a discussion:

We came up with two different approaches for better NUMA support in Xen:
1.) Guest NUMA support: spread a guest's resources (CPUs and memory) over several nodes and propagate the appropriate topology to the guest. The first part of this is in the patches I sent recently to the list (PV support is following, bells and whistles like automatic placement will follow, too.).
- The guest OS has better means to deal with the NUMA setup, it can more easily migrate _processes_ among the nodes (Xen-HV can only migrate whole domains).
- Changes to Xen are relatively small.
- There is no limit for the guest resources, since they can use more resources than there are on one node. - If guests are well spread over the nodes, the system is more balanced even if guests are destroyed and created later.
- The guest has to support NUMA. This is not true for older guests (Win2K, older Linux). - The guest's workload has to fit NUMA. If the guests tasks are merely parallelizable or use much shared memory, they cannot take advantage of NUMA and will degrade in performance. This includes all single task problems.

In general this approach seems to fit better with smaller NUMA nodes and larger guests.

2.) Dynamic load balancing and page migration: create guests within one NUMA node and distribute all guests across the nodes. If the system becomes imbalanced, migrate guests to other nodes and copy (at least part of) their memory pages to the other node's local memory.
- No guest NUMA support necessary. Older as well a recent guests should run fine. - Smaller guests don't have to cope with NUMA and will have 'flat' memory available. - Guests running on separate nodes usually don't disturb each other and can benefit from the higher distributed memory bandwidth.
- Guests are limited to the resources available on one node. This applies for both the number of CPUs and the amount of memory. - Costly migration of guests. In a simple implementation we'd use live migration, which requires the whole guest's memory to be copied before the guest starts to run on the other node. If this whole move proves to be unnecessary a few minutes later, all this was in vain. A more advanced implementation would do the page migration in the background and thus can avoid this problem, if only the hot pages are migrated first. - Integration into Xen seems to be more complicated (at least for the more ungifted hackers among us).

This approach seems to be more reasonable if you have larger nodes (for instance 16 cores) and smaller guests (the more usual case nowadays?)

After some discussion we came to the conclusion that both approaches should be implemented. I want to put this to the list and am looking forward to any feedback.


Andre Przywara
AMD-Operating System Research Center (OSRC), Dresden, Germany
Tel: +49 351 277-84917
----to satisfy European Law for business letters:
AMD Saxony Limited Liability Company & Co. KG
Sitz (Geschäftsanschrift): Wilschdorfer Landstr. 101, 01109 Dresden, Deutschland
Registergericht Dresden: HRA 4896
vertretungsberechtigter Komplementär: AMD Saxony LLC (Sitz Wilmington, Delaware, USA)
Geschäftsführer der AMD Saxony LLC: Dr. Hans-R. Deppe, Thomas McCoy

Xen-devel mailing list