Re: [PATCH 2/5] atomic: introduce atomic_inc_not_zero
From: Roman Zippel
Date: Fri Sep 16 2005 - 20:17:52 EST
Hi,
On Thu, 15 Sep 2005, Nick Piggin wrote:
> Roman: any ideas about what you would prefer? You'll notice
> atomic_inc_not_zero replaces rcuref_inc_lf, which is used several times
> in the VFS.
In the larger picture I'm not completely happy with these scalibilty
patches, as they add extra overhead at the lower end. On a UP system in
general nothing beats:
spin_lock();
if (*ptr)
ptr += 1;
spin_unlock();
The main problem is here that the atomic functions are used in two basic
situation:
1) interrupt synchronization
2) multiprocessor synchronization
The atomic functions have to assume both, but on UP systems it often is
a lot cheaper if they don't have to synchronize with interrupts. So
replacing a spinlock with a few atomic operations can hurt UP performance.
bye, Roman
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