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Re: [PATCH] migration: Move bitmap_mutex out of migration_bitmap_clear_d

From: Peter Xu
Subject: Re: [PATCH] migration: Move bitmap_mutex out of migration_bitmap_clear_dirty()
Date: Wed, 7 Jul 2021 20:21:22 -0400

On Wed, Jul 07, 2021 at 11:25:50PM +0000, Wang, Wei W wrote:
> On Thursday, July 8, 2021 12:45 AM, Peter Xu wrote:
> > On Wed, Jul 07, 2021 at 12:45:32PM +0000, Wang, Wei W wrote:
> > > Btw, what would you think if we change mutex to QemuSpin? It will also 
> > > reduce
> > the overhead, I think.
> > 
> > As I replied at the other place, the bottleneck we encountered is the lock 
> > taking
> > not sleeping, so I'm afraid spinlock will have the same issue. Thanks,
> I suspect the overhead you observed might be caused by the syscalls for 
> mutex. Per-page syscall might be too much.
> If possible, you could have another test of the 3GB guest migration using 
> QemuSpin.

I'm a bit confused.. Why taking a mutex with no contention would need a syscall?

Please feel free to strace on a general version glibc impl of pthread mutex
which qemu uses, I believe it didn't need to.


Peter Xu

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