Re: [Patch 2/3] fast VMA recycling

[Andi Kleen <ak@suse.de>]

On Thursday 23 February 2006 11:15, Arjan van de Ven wrote:
> On Thu, 2006-02-23 at 11:05 +0100, Andi Kleen wrote:
> > On Thursday 23 February 2006 10:48, Arjan van de Ven wrote:
> > > On Thu, 2006-02-23 at 10:42 +0100, Andi Kleen wrote:
> > > > On Thursday 23 February 2006 10:30, Arjan van de Ven wrote:
> > > > > This patch adds a per task-struct cache of a free vma. 
> > > > > 
> > > > > In normal operation, it is a really common action during userspace mmap 
> > > > > or malloc to first allocate a vma, and then find out that it can be merged,
> > > > > and thus free it again. In fact this is the case roughly 95% of the time.
> > > > > 
> > > > > In addition, this patch allows code to "prepopulate" the cache, and
> > > > > this is done as example for the x86_64 mmap codepath. The advantage of this
> > > > > prepopulation is that the memory allocation (which is a sleeping operation
> > > > > due to the GFP_KERNEL flag, potentially causing either a direct sleep or a 
> > > > > voluntary preempt sleep) will happen before the mmap_sem is taken, and thus 
> > > > > reduces lock hold time (and thus the contention potential)
> > > > 
> > > > The slab fast path doesn't sleep. 
> > > 
> > > it does via might_sleep()
> > 
> > Hmm? That shouldn't sleep.
> 
> see voluntary preempt.

Only when its time slice is used up but then it would sleep a bit later 
in user space. But it should be really a unlikely case and nothing
to optimize for.

> 
> > If it takes any time in a real workload then it should move into DEBUG_KERNEL
> > too. But I doubt it. Something with your analysis is wrong.
> 
> well I'm seeing contention; and this is one of the things that can be
> moved out of the lock easily, and especially given the high recycle rate
> of these things... looks worth it to me.

I think you need a better analysis of what is actually happening
instead of trying all kind of weird quick fragile hacks.

-Andi

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