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Re: [libvirt] [PATCH] qemu: lock: unlock vm during qemuBuildCommandLine



On Thu, Apr 23, 2015 at 02:02:25PM +0200, Michal Privoznik wrote:
> On 23.04.2015 13:32, zhang bo wrote:
> > On 2015/4/23 19:06, Daniel P. Berrange wrote:
> > 
> >> On Thu, Apr 23, 2015 at 07:00:21PM +0800, zhang bo wrote:
> >>> The reason for the problem is that:
> >>> 1 guestA locks vm while creating each tapDev(virNetDevTapCreate) in qemuBuildCommandLine(), for 10seconds
> >>> 2 guestB calls qemuMigrationPrepareAny->*virDomainObjListAdd* to get its vm object, which locks 'doms'
> >>>   and waits for the vm lock.
> >>> 3 doms will be locked until guestA unlock its vm, we say that's 10 seconds.
> >>> 4 guestC calls qemuDomainMigrateFinish3->virDomainObjListFindByName, which tries to lock doms. because it's
> >>
> >> Ok, this is the real core problem - FindByName has a bad impl that requires
> >> iterating over every single guest. Unfortunately due to the design of the
> >> migration API we can't avoid this call, but we could add a second hash table
> >> of name -> virDomainObj so we make it O(1) and lock-less.
> >>
> 
> Agreed. This will solve more than this.
> 
> > 
> > I got a question: shall we add an object (similar to doms) and lock it while searching the vm
> > in the new hash table? If so, the problem may still exist.
> 
> No. Currently, we use a hash table, where domain's UUID is the key, and
> pointer to internal domain representation (or domain object as it's
> called) is the value. Looking up a key in hash table is O(1).
> 
> However, if we look up a domain by name, it's practically lookup by
> value, because domain name is stored in the domain object. So we have to
> iterate over all hash table items, and lock each one just to compare.
> However, if an object is locked already, we must wait until it's
> unlocked again. During this wait we still hold the hash table lock.
> 
> I don't think this is trivial though. So let me see if I can provide
> some patches.

I'd just create a 2nd hash table in virDomainObjList object, that either
maps name -> virDomainObjPtr, or maps name -> uuid.

Regards,
Daniel
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