Re: kernel stack size

[Manfred Spraul]

Steven Rostedt wrote:

> > Have you benchmarked your own memory manager?
> > kmalloc(1024, GFP_KERNEL) is something like 17 instructions on i386 
> > uniprocessor.
> >    
>
> Where did you get that? I'm looking at the assembly of it right now and
> it's much larger than 17 instructions. Not to mention that it calls the
> slab functions which might have to invoke the buddy system.
>
>  
Have you looked at kmem_cache_alloc? kmalloc(1024, GFP_KERNEL) is 
compile-time replaced with the appropriate kmem_cache_alloc call. And 
the fast path within kmem_cache_alloc is 17 instructions long. Best 
case: uniprocessor, no regparams. Unfortunately with cli and popfd, thus 
something like 35 cpu cycles on an Athlon 64.

> I haven't clocked the speed of sem compared to kmalloc.
> But I would think that the sem functions are still quicker.
>
>  
Yes - sem or spin locks are quicker as long as no cache line transfers 
are necessary. If the semaphore is accessed by multiple cpus, then 
kmalloc would be faster: slab tries hard to avoid taking global locks. 
I'm not speaking about contention, just the cache line ping pong for 
acquiring a free semaphore.

--
   Manfred
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