Re: [PATCH 0/4] Really lazy fpu

From: Avi Kivity
Date: Mon Jun 14 2010 - 03:47:47 EST

On 06/13/2010 11:45 PM, Valdis.Kletnieks@xxxxxx wrote:
On Sun, 13 Jun 2010 18:03:43 +0300, Avi Kivity said:
Currently fpu management is only lazy in one direction. When we switch into
a task, we may avoid loading the fpu state in the hope that the task will
never use it. If we guess right we save an fpu load/save cycle; if not,
a Device not Available exception will remind us to load the fpu.

However, in the other direction, fpu management is eager. When we switch out
of an fpu-using task, we always save its fpu state.
Does anybody have numbers on how many clocks it takes a modern CPU design
to do a FPU state save or restore?

320 cycles for a back-to-back round trip. Presumably less on more modern hardware, more if uncached, more on even more modern hardware that has the xsave header (8 bytes) and ymm state (256 bytes) in addition.

I know it must have been painful in the
days before cache memory, having to make added trips out to RAM for 128-bit
registers. But what's the impact today?

I'd estimate between 300 and 600 cycles depending on the factors above.

(Yes, I see there's the potential
for a painful IPI call - anything else?)

The IPI is only taken after a task migration, hopefully a rare event. The patchset also adds the overhead of irq save/restore. I think I can remove that at the cost of some complexity, but prefer to start with a simple approach.

Do we have any numbers on how many saves/restores this will save us when
running the hypothetical "standard Gnome desktop" environment?

The potential is in the number of context switches per second. On a desktop environment, I don't see much potential for a throughput improvement, rather latency reduction from making the crypto threads preemptible and reducing context switch times.

Servers with high context switch rates, esp. with real-time preemptible kernels (due to threaded interrupts), will see throughput gains. And, of course, kvm will benefit from not needing to switch the fpu when going from guest to host userspace or to a host kernel thread (vhost-net).

How common
is the "we went all the way around to the original single FPU-using task" case?

When your context switch is due to an oversubscribed cpu, not very common. When it is due to the need to service an event and go back to sleep, very common.

error compiling committee.c: too many arguments to function

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