Re: [Patch 2/3] fast VMA recycling

[Andi Kleen]

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. And if you're short of memory 
then you probably have other issues.
 
> The cache is only allowed to be accessed for "current", and not from IRQ
> context. This allows for lockless access, making it a cheap cache.
> 
> One could argue that this should be a generic slab feature (the preloading) but
> that only gives some of the gains and not all, and vma's are small creatures,
> with a high "recycling" rate in typical cases.

Numbers numbers numbers. What workload? How much did it help? 

Also looks like a very narrow purpose hard to maintain fragile hack to me.

-Andi
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