From mboxrd@z Thu Jan 1 00:00:00 1970 From: Jeremy Fitzhardinge Subject: Re: [PATCH 2 of 8] libxl: introduce libxl_set_relative_memory_target Date: Wed, 01 Sep 2010 14:17:39 -0700 Message-ID: <4C7EC2F3.3050704@goop.org> References: <19581.15132.637644.952724@mariner.uk.xensource.com 4C7E94F0.5050802@goop.org> <43844cf2-4c99-448c-a98c-3100baae6dcc@default> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <43844cf2-4c99-448c-a98c-3100baae6dcc@default> List-Unsubscribe: , List-Post: List-Help: List-Subscribe: , Sender: xen-devel-bounces@lists.xensource.com Errors-To: xen-devel-bounces@lists.xensource.com To: Dan Magenheimer Cc: xen-devel@lists.xensource.com, Ian Jackson , Stefano Stabellini List-Id: xen-devel@lists.xenproject.org On 09/01/2010 01:03 PM, Dan Magenheimer wrote: > Indeed, that's what selfballooning does. The xenstore watch > is irrelevant for selfballooning (though the watch also can be used > asynchronously for backwards compatibility). There's no mechanism to make the balloon driver ignore the target watch, so any updates to xenstore will update the driver's target. > IMHO, attempts to do memory load balancing externally (e.g. setting > a memory target from tools in dom0) are doomed to failure. There > was a discussion of memory "rightsizing" at the recent Linux MM summit; > this is an almost impossible problem even within a single kernel, > though there were heuristics discussed as to how to approach it... > and a better understanding about why in-kernel tmem-ish functionalities > like cleancache and frontswap are useful for mitigating the problems > that occur when rightsizing is approximated. > [...] > So, frankly, I think the "xm memset" functionality is largely > useless, but agree that it should be maintained in xl for backwards > compatibility. But trying to comingle the concepts of maxmem > and target is a bad idea. In the general case I think you're probably right (I can't see it being useful in a VPS hosting service, for example), but there are definitely special cases where it is useful. Squashing down existing domains to make room for a new one, for example, or more app-specific uses. Giving domains some real incentive to be economical with memory would probably change the landscape a lot. But I don't think there's a real solution without knowing the specifics of that incentive. J