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Re: [Xen-devel] Ticket spinlocks and MP guests

To: Espen Skoglund <espen.skoglund@xxxxxxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Ticket spinlocks and MP guests
From: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Date: Thu, 14 Feb 2008 15:23:55 +0000
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Thread-topic: [Xen-devel] Ticket spinlocks and MP guests
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On 14/2/08 13:43, "Espen Skoglund" <espen.skoglund@xxxxxxxxxxxxx> wrote:

> Do people feel that the new ticket spinlocks should raise any
> concerns, or do typical Xen SMP guest workloads remain largely
> unaffected by the new locking scheme?

That's a good question which I do not think can be answered without taking
some measurements. If it's an issue we might consider pv_ops'ifying
spinlocks to turn them into sleeping locks (this would be easy these days,
since spin_lock() and spin_unlock() are not inlined). I have some neat ideas
about how that might work requiring no extra space for spinlock_t and no
modifications at the hypervisor interface.

I suppose it depends whether there are any hot spinlocks in the kernel these
days. I know things have improved a lot in recent years with things like
RCU. Most spinlock regions are now pretty short.

 -- Keir

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