Hello, I've tested the scenario with restoring too many domains having lock commented out. It turns out that in case when there is no memory left for XEN to allocate, all pending /xl restore/ commands will simply fail with an internal error.
As far as I understand, it's not going to do any damage unless I request too much memory? If so, then there is some place for improvement, i.e. to make xl acquire the lock, allocate memory, release the lock and then start restoring the domain state. Maybe I could provide some pull request if I would manage to implement such behavior. Best regards, Michał Leszczyński On 10.06.2019 11:41, Anthony PERARD wrote: > On Fri, Jun 07, 2019 at 02:06:30PM +0200, Michał Leszczyński wrote: >> Hello, > Hi, > >> when either "xl restore" or "xl create" command is invoked, a global lock is >> acquired here: >> >> https://github.com/xen-project/xen/blob/master/tools/xl/xl_vmcontrol.c#L876 >> >> I'm trying to figure out what is the exact importance of this lock? Is it >> really critical for XL operation? I have a pretty powerful machine on which >> I want to run a few dozen of short-lived VMs. This lock is seriously slowing >> me down, not letting to restore more than one domain at once. Turns out that >> everything still works fine (and much faster) when I comment-out the lock in >> the abovementioned place, but I'm not sure if it's a correct impression. >> >> Does anyone know if there is a situation in which Xen would screw up without >> this lock? > Here is the reason for the lock: > https://xenbits.xen.org/gitweb/?p=xen.git;a=commit;h=ea4dce89d478d62341cd2f9d8944e215f7086144 > > xl: free memory before building a domain > Free the needed amount of memory before proceeding with the domain > build. > Use a filelock to prevent other xl instances from conflicting during > this operation. > > So there are probably configurations where the lock isn't useful, or > there are better ways to reserve memory for domain creation. > -- Pozdrawiam Michał Leszczyński CERT Polska/NASK +48 532 461 124
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