Locking in xl

Locking in xl

[Neil Sikka]

Hello,
I see that in the xl binary in xen 4.13.0, the acquire_lock() and
release_lock() functions are only called from create_domain() in
xl_vmcontrol.c, so I assume the lock provides inter-PROCESS
synchronization in the case that multiple instances of xl are running
and creating multiple domains concurrently.

However, this lock causes a bottleneck in the case that an xl restore
process is restoring a DomU with a lot of memory. While the large
amount of memory is being copied from the checkpoint file on disk to
the physical machine's RAM, all other VM creation requests on the
system are starved, leading to a performance loss. When I removed the
lock, my testing of simultaneous creation of 2 DomU's concurrently
worked and I did not see any issues.

Does anyone know what shared resource these locks are guarding? Maybe
we should be making the lock more granular.

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