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Re: [Xen-devel] [PATCH] switch rangeset's lock to rwlock

Jan Beulich wrote:
On 30.09.14 at 22:53,<keir.xen@xxxxxxxxx>  wrote:
Do the searches ever get long enough that a read lock helps? If any of
the rangesets is getting large and making the searches slow then it
would be quite easy to switch from linked list to red-black tree?

As noted elsewhere, even very brief locking periods can cause
convoys for many-vCPU guests. One case where we observe
this is hvm_get_guest_time_fixed() (without clear route for
mitigation as converting to rw lock is not an option here, and I
didn't get around to try out whether eliminating the lock
altogether in favor of atomic CPU operations would make this
any better).

Given how rarely most of these rangesets get updated, it would be nice to let the guest itself have lock-free access at the expense of having to pause it to make modifications. Although, does a guest ever modify any of its own rangesets? I'm not sure that ever happens.

 -- Keir


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