On 8/5/22 09:35, Leonardo Bras wrote:
1 - Spin-Locking in mark_page_dirty_in_slot():
I understand this function happens a lot in the guest and should probably
be as fast as possible, so introducing a lock here seems
counter-productive, but to be fair, I could not see it any slower than a
couple cycles in my current setup (x86_64 machine).
Maybe too small workload? 32 vcpus at 8 GB/s would mean 256 MB/s/vCPU,
i.e. 16384 pages/second *at most*. That might not create too much
contention.
One possibility here is to use a global (for all VMs)
percpu_rw_semaphore, or perhaps even RCU. The write critical section is
so short that it could be a win nevertheless.
However...
2 - Qemu will use the 'manual_dirty_log_protect'
I understand that more recent versions qemu will use
'manual_dirty_log_protect' when available, so this approach will not
benefit this use case, which is quite common.
A counter argument would be: there are other hypervisors that could benefit
from it, and that is also applicable for older qemu versions.
... that was my first thought indeed. I would just consider the old API
to be legacy and not bother with it. Mostly because of the ability to
clear a small part of the bitmap(*), and the initially-all-set
optimization, manual dirty log ought to be superior even if
CLEAR_DIRTY_LOG has to use atomics.
- I am also trying to think on improvements for the
'manual_dirty_log_protect' use case, which seems to be very hard to
improve. For starters, using the same approach to remove the atomics
does not seem to cause any relevant speedup.
Yes, there are two issues:
1) CLEAR_DIRTY_LOG does not clear all bits, only those passed in by
userspace. This means that the inactive bitmap still has some bits set
2) the ability to clear part of the bitmap makes it hard to do a
wholesale switch in CLEAR_DIRTY_LOG; this is the dealbreaker
Thanks,
Paolo
(*) for the old API, that requires a workaround of using multiple small
memslots, e.g. 1-32G in size
