Hello, I''m trying to get into the internals of xen and especially the balloon driver. During the time I dealt with that, several questions came up: * Does (and if yes, how) the balloon driver in an guest OS have to be activated? * Does the automatic release of memory from a guest OS to Xen already work? * Is the limit of 100 Guest OSs (as mentioned on http://en.wikipedia.org/wiki/Xen_%28virtual_machine_monitor%29 ) really hardcoded somewhere or is it more a practical value (or does the limit not exist)? Thanks in advance & regards, Jonas Appel _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
Jonas Appel wrote:>Hello, > >I''m trying to get into the internals of xen and especially the balloon >driver. During the time I dealt with that, several questions came up: > >* Does (and if yes, how) the balloon driver in an guest OS have to be >activated? > >No. In Linux, it''s a normal driver that''s loaded and initialized during system startup.>* Does the automatic release of memory from a guest OS to Xen already >work? > >It depends on what you mean by automatic. I''m not sure anyone has ever posted an automated memory balancer.>* Is the limit of 100 Guest OSs (as mentioned on >http://en.wikipedia.org/wiki/Xen_%28virtual_machine_monitor%29 ) really >hardcoded somewhere or is it more a practical value (or does the limit >not exist)? > >It''s a design goal, not a limit. I''ve heard of folks actually getting more than 100 domains going at once (although the domains were not full-blown linux instances). Regards, Anthony Liguori>Thanks in advance & regards, > >Jonas Appel > > >_______________________________________________ >Xen-devel mailing list >Xen-devel@lists.xensource.com >http://lists.xensource.com/xen-devel > > >_______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
> I''m trying to get into the internals of xen and especially the balloon > driver. During the time I dealt with that, several questions came up:Excellent! The more people who know about this stuff (and preferably hack on it ;-) the better.> * Does (and if yes, how) the balloon driver in an guest OS have to be > activated?It''s purely manual right now: you set a target memory allocation and the balloon driver will try and resize the guest to fit. The target is set by an interface in /proc, or by a message on the Xen control interface (soon to be replaced by a value in the xenstore registry).> * Does the automatic release of memory from a guest OS to Xen already > work?Nope, we''d like some kind of daemon to run in dom0 and balloon guests according to their need. This should probably include some activity statistics reported by the guests, so the daemon can figure out which needs more memory. Swap statistics might give a good idea of which guests needed memory.> * Is the limit of 100 Guest OSs (as mentioned on > http://en.wikipedia.org/wiki/Xen_%28virtual_machine_monitor%29 ) really > hardcoded somewhere or is it more a practical value (or does the limit > not exist)?You should be able to start several hundred guests before hitting any hard coded limits (e.g. number of event channels - I''m told there''s a plan to remove this restriction). Before you get to that stage, most systems will run out of hardware oomph to run so many guests *and* still do something useful. I think Ian started somewhere over 100 guests one time and IIRC he ran out of disk images before hitting any limits :-) Any limits will likely be addressed, since people are a) starting to use very large machines with Xen (Unisys are running it on 32 CPU boxes) and b) using cunning tricks to get extremely large numbers of guests running (I''ve heard "thousands" mentioned). Cheers, Mark _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
> It depends on what you mean by automatic. I''m not sure anyone has ever > posted an automated memory balancer.One fine day, someone should write this. It''d allow you to trade off a bit of isolation for a bit of statistical multiplexing in the case where your domains don''t have a paranoid mistrust of each other. VMware ESX sets a minimum, a maximum and a target percentage share for each machine. It then does sampling of the domains to find out how much of their memory they''re *really* using. If they''re not using all of it, they get told to give it back using the balloon driver and it''s apportioned to domains that are using their memory more heavily. This sort of thing should carry fairly straightforwardly across to Xen. We could simply get domains to report their current utilisation through the store without needing to do sampling behind their back. Could be a fun project for someone looking to get into Xen stuff (yes, that''s an unsubtle hint to the world at large :-) ). Cheers, Mark _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
On Tue, 2005-08-02 at 01:48 +0100, Mark Williamson wrote:> > * Is the limit of 100 Guest OSs (as mentioned on > > http://en.wikipedia.org/wiki/Xen_%28virtual_machine_monitor%29 ) really > > hardcoded somewhere or is it more a practical value (or does the limit > > not exist)? > > You should be able to start several hundred guests before hitting any hard > coded limits (e.g. number of event channels - I''m told there''s a plan to > remove this restriction). Before you get to that stage, most systems will > run out of hardware oomph to run so many guests *and* still do something > useful. I think Ian started somewhere over 100 guests one time and IIRC he > ran out of disk images before hitting any limits :-)At least on my machine, with current unstable, I cannot create more than 140 *completely empty* domains (just created, no resources assigned); attempting to create more at that point gives an out-of-memory error (which I believe is referring to the Xen hypervisor kernel memory). Perhaps there is some switch I haven''t seen which allows increasing the amount of memory allocated to the Xen kernel? It would also be nice if domains had far less overhead until resources were allocated to them. - Josh Triplett _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
> At least on my machine, with current unstable, I cannot > create more than 140 *completely empty* domains (just > created, no resources assigned); attempting to create more at > that point gives an out-of-memory error (which I believe is > referring to the Xen hypervisor kernel memory). > Perhaps there is some switch I haven''t seen which allows > increasing the amount of memory allocated to the Xen kernel? > It would also be nice if domains had far less overhead until > resources were allocated to them.Sounds like you''re running out of Xen heap, which is currently statically sized. Each domain ''shell'' allocates something like 64KB of storage. I''m not sure we''ll get around to changing this anytime soon on, at least on x86_32. On x86_64 there''s no need for there to be a distinction between domain and xen heap pages, hence the xen heap can be dynamically sized (at least in the bottom 4GB) Ian _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel
On 4 Aug 2005, at 23:03, Josh Triplett wrote:> At least on my machine, with current unstable, I cannot create more > than > 140 *completely empty* domains (just created, no resources assigned); > attempting to create more at that point gives an out-of-memory error > (which I believe is referring to the Xen hypervisor kernel memory). > Perhaps there is some switch I haven''t seen which allows increasing the > amount of memory allocated to the Xen kernel? It would also be nice if > domains had far less overhead until resources were allocated to them.You run out of Xen heap. We won;t be able to make any more available on x86_32, but there is a boot parameter to x86/64 Xen to set the amount of Xen heap, in megabytes. -- Keir _______________________________________________ Xen-devel mailing list Xen-devel@lists.xensource.com http://lists.xensource.com/xen-devel