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Date: Mon, 3 Mar 2008 14:14:42 +1100 From: Nick Piggin <nickpiggin@...oo.com.au> To: Christoph Lameter <clameter@....com> Cc: Peter Zijlstra <a.p.zijlstra@...llo.nl>, Eric Dumazet <dada1@...mosbay.com>, "David S. Miller" <davem@...emloft.net>, Andrew Morton <akpm@...ux-foundation.org>, linux kernel <linux-kernel@...r.kernel.org>, netdev@...r.kernel.org, "Zhang, Yanmin" <yanmin_zhang@...ux.intel.com> Subject: Re: [PATCH] alloc_percpu() fails to allocate percpu data 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 like > > 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? > > Tradition.... Its irks me as well. > > > Things like this are just going to lead to very difficult to track > > performance problems. Possibly correctness problems in rare cases. > > > > There could be another flag for "maybe align". > > SLAB_HWCACHE_ALIGN *is* effectively a maybe align flag given the above > 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. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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