Messages in this thread | | | Date | Tue, 13 Dec 2005 22:38:54 +0100 | From | Eric Dumazet <> | Subject | Re: [PATCH] Cpuset: rcu optimization of page alloc hook |
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Paul Jackson a écrit : > Eric wrote: > >>If this variable is not frequently used, why then define its own cache ? >> >>Ie why not use kmalloc() and let kernel use a general cache ? > > > This change from kmalloc() to a dedicated slab cache was made just a > couple of days ago, at the suggestion of Andi Kleen and Nick Piggin, in > order to optimize out a tasklock spinlock from the primary code path > for allocating a page of memory. > > Indeed, this email thread is the thread that presented that patch. > > By using a dedicated slab cache, I was able to make an unusual use of > Hugh Dicken's SLAB_DESTROY_BY_RCU implementation, and access a variable > inside the cpuset structure safely, even after that cpuset structure > might have been asynchronously free'd. What I read from that variable > might well be garbage, but at least the slab would not have freed that > page of memory entirely, inside my rcu_read_lock section.
OK, I'm afraid I cannot comment on this, this is too complex for me :)
> > Since all I needed was to edge trigger on the condition that the > contents of a variable changed since last read, that was sufficient. > > >>On a 32 CPUS machine, a kmem_create() costs a *lot* of ram. > > > Hmmm ... if 32 is bad, then what does it cost for say 512 CPUs?
You dont want to know :)
struct kmem_cache itself will be about 512*8 + some bytes then for each cpu a 'struct array_cache' will be allocated (count 128 bytes minimum each, it depends on various factors (and sizeof(void*) of course)
So I would say about 80 K Bytes at a very minimum.
> > And when is that memory required? On many systems, that will have > cpusets CONFIG_CPUSET enabled, but that are not using cpusets, just > the kmem_cache_create() will be called to create cpuset_cache, but > -no- kmem_cache_alloc() calls done. On those systems using cpusets, > there might be one 'struct cpuset' allocated per gigabyte of ram, as a > rough idea. > > Can you quantify "costs a *lot* of ram" ? > > I suppose that I could add a little bit of logic that avoided the > initial kmem_cache_create() until needed by actual cpuset usage on the > system (on the first cpuset_create(), the first time that user code > tries to create a cpuset). In a related optimization, I might be able > to avoid -even- the rcu_read_lock() guards on systems not using > cpusets (never called cpuset_create() since boot), reducing that guard > to a simple comparison of the current tasks cpuset pointer with the > pointer to the one statically allocated global cpuset, known as the root > cpuset. Actually, that last opimization would benefit any task still > in the root cpuset, even after other cpusets had been dynamically > created. > > Or, if using the slab cache was still too expensive for this use, I > could perhaps make a more conventional use of RCU, to guard the kfree() > myself, instead of making this unusual use of SLAB_DESTROY_BY_RCU. I'd > have to learn more about RCU to know how to do that, or even it made > sense. >
Thank you for this details.
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