Re: [RFC PATCH]: Align large allocations to cacheline

[Rich Felker <dalias at libc dot org>]

On Wed, Oct 29, 2014 at 01:43:54PM -0000, Wilco Dijkstra wrote:
> This patch aligns allocations of large blocks to a cacheline on ARM and AArch64. The main goal is to
> reduce performance variations due to random alignment choices, however it improves performance on
> several benchmarks as well. SPECFP2000 improves by ~1.5%.
> 
> Any comments?

It seems like this would pathologically increase fragmentation: when a
tiny block is split off the beginning to align a large allocation,
isn't it likely that this tiny chunk (rather than some other tiny
chunk already in the appropriate-sized free bin prior to the large
allocation) will be used to satisfy a tiny request, which may end up
being long-lived?

memalign-type operations are known to be a major problem for
fragmentation. See this bug report:

https://sourceware.org/bugzilla/show_bug.cgi?id=14581

A smarter approach might be to round up the _sizes_ of large
allocations so that the next available address after the allocated
block is nicely aligned. This will tend to make _future_ allocations
aligned, even if the first one isn't, and won't leave any free gaps
that could contribute to fragmentation.

Rich