Nick Piggin a écrit :
If an allocator knows exactly the lifetime of its page, then there is no
need to do refcounting or the final put_page_zestzero (atomic op + mem
barriers).
This is probably not worthwhile for most cases, but slab did strike me
as a potential candidate (however the complication here is that some
code I think uses the refcount of underlying pages of slab allocations
eg nommu code). So it is not a complete patch, but I wonder if anyone
thinks the savings might be worth the complexity?
Is there any particular code that is really heavy on slab allocations?
That isn't mostly handled by the slab's internal freelists?
Hi Nick
After reading your patch, I have some crazy idea.
The atomic op + mem barrier you want to avoid could be avoided more generally
just by changing atomic_dec_and_test(atomic_t *v).
If the current thread is the last referer (refcnt = 1), then it can safely set
the value to 0 because no other CPU can be touching the value (or else there
must be a bug somewhere, as the 'other cpu' could touch the value just after
us and we could free an object still in use by 'other cpu'
Something like :
--- include/asm-i386/atomic.h.orig 2006-01-25 12:11:46.000000000 +0100
+++ include/asm-i386/atomic.h 2006-01-25 12:13:07.000000000 +0100
@@ -130,6 +130,13 @@
printk("BUG: atomic counter underflow at:\n");
dump_stack();
}
+#ifdef CONFIG_SMP
+ /* avoid an atomic op if we are the last user of this atomic */
+ if (atomic_read(v) == 1) {
+ atomic_set(v, 0); /* not a real atomic op on most machines */
+ return 1;
+ }
+#endif
__asm__ __volatile__(
LOCK_PREFIX "decl %0; sete %1"
:"=m" (v->counter), "=qm" (c)
Thank you
Eric
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