Re: [PATCH] migration: Move bitmap_mutex out of migration_bitmap_clear_dirty()

[Peter Xu]

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.

Thanks,

-- 
Peter Xu