Re: Enhancement for PLE handler in KVM

[Paolo Bonzini <pbonzini@redhat.com>]

Il 03/03/2014 19:24, Li, Bin (Bin) ha scritto:
> Hello, all.
>
> The PLE handler attempts to determine an alternate vCPU to schedule.  In
> some cases the wrong vCPU is scheduled and performance suffers.
>
> This patch allows for the guest OS to signal, using a hypercall, that
> it's starting/ending a critical section.  Using this information in the
> PLE handler allows for a more intelligent VCPU scheduling determination
> to be made.  The patch only changes the PLE behaviour if this new
> hypercall mechanism is used; if it isn't used, then the existing PLE
> algorithm continues to be used to determine the next vCPU.
>
> Benefit from the patch:
>  -  the guest OS real time performance being significantly improved when
> using hyper call marking entering and leaving guest OS kernel state.
>  - The guest OS system clock jitter measured on on Intel E5 2620 reduced
> from 400ms down to 6ms.
>  - The guest OS system lock is set to a 2ms clock interrupt. The jitter
> is measured by the difference between dtsc() value in clock interrupt
> handler and the expectation of tsc value.
>  - detail of test report is attached as reference.

This patch doesn't include the corresponding guest changes, so it's not 
clear how you would use it and what the overhead would be: a hypercall 
is ~30 times slower than an uncontended spin_lock or spin_unlock.

In fact, performance numbers for common workloads are useful too.

Have you looked at the recent "paravirtual ticketlock"?  It does roughly 
the opposite as this patch: the guest can signal when it's been spinning 
too much, and the host will schedule it out (which hopefully accelerates 
the end of the critical section).

Paolo



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