On 05/03/2012 05:10 AM, Marcelo Tosatti wrote: > On Wed, May 02, 2012 at 01:39:51PM +0800, Xiao Guangrong wrote: >> On 04/29/2012 04:50 PM, Takuya Yoshikawa wrote: >> >>> On Fri, 27 Apr 2012 11:52:13 -0300 >>> Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote: >>> >>>> Yes but the objective you are aiming for is to read and write sptes >>>> without mmu_lock. That is, i am not talking about this patch. >>>> Please read carefully the two examples i gave (separated by "example)"). >>> >>> The real objective is not still clear. >>> >>> The ~10% improvement reported before was on macro benchmarks during live >>> migration. At least, that optimization was the initial objective. >>> >>> But at some point, the objective suddenly changed to "lock-less" without >>> understanding what introduced the original improvement. >>> >>> Was the problem really mmu_lock contention? >>> >> >> >> Takuya, i am so tired to argue the advantage of lockless write-protect >> and lockless O(1) dirty-log again and again. > > His point is valid: there is a lack of understanding on the details of > the improvement. > Actually, the improvement of lockless is that it can let vcpu to be parallel as possible. >From the test result, lockless gains little improvement for unix-migration, in this case, the vcpus are almost idle (at least not busy). The large improvement is from dbench-migration, in this case, all vcpus are busy accessing memory which is write-protected by dirty-log. If you enable page-fault/fast-page-fault tracepoints, you can see huge number of page fault from different vcpu during the migration. > Did you see the pahole output on struct kvm? Apparently mmu_lock is > sharing a cacheline with read-intensive memslots pointer. It would be > interesting to see what are the effects of cacheline aligning mmu_lock. > Yes, i see that. In my test .config, i have enabled CONFIG_DEBUG_SPINLOCK/CONFIG_DEBUG_LOCK_ALLOC, mmu-lock is not sharing cacheline with memslots. That means it is not a problem during my test. (BTW, pahole can not work on my box, it shows: ...... DW_AT_<0x3c>=0x19 DW_AT_<0x3c>=0x19 DW_AT_<0x3c>=0x19 die__process_function: DW_TAG_INVALID (0x4109) @ <0x12886> not handled! ) If we reorganize 'struct kvm', i guess it is good for kvm but it can not improve too much for migration. :) -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
