On Thursday 28 February 2008 06:44, Christoph Lameter wrote:On Sat, 23 Feb 2008, Nick Piggin wrote:What I don't understand is why the slab allocators have something likeTradition.... Its irks me as well.
this in it:
if ((flags & SLAB_HWCACHE_ALIGN) &&
size > cache_line_size() / 2)
return max_t(unsigned long, align, cache_line_size());
If you ask for HWCACHE_ALIGN, then you should get it. I don't
understand, why do they think they knows better than the caller?
Things like this are just going to lead to very difficult to trackSLAB_HWCACHE_ALIGN *is* effectively a maybe align flag given the above
performance problems. Possibly correctness problems in rare cases.
There could be another flag for "maybe align".
code.
If we all agree then we could change this to have must have semantics? It
has the potential of enlarging objects for small caches.
SLAB_HWCACHE_ALIGN has an effect that varies according to the alignment
requirements of the architecture that the kernel is build on. We may be in
for some surprises if we change this.
I think so. If we ask for HWCACHE_ALIGN, it must be for a good reason.
If some structures get too bloated for no good reason, then the problem
is not with the slab allocator but with the caller asking for
HWCACHE_ALIGN.