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



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.
> 

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.

>>   now locked by guestB, guestC blocks here, and it can't be unpaused for at least 10 seconds.
>> 5 then comes to guestD, guestE, guestF, etc, the downtime will be added up, to even 50 seconds or more.
>> 6 the command 'virsh list' is blocked as well.
>>
>> Thus, we think the problem must be solved.
> 
> Regards,
> Daniel




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