On Thu, 12 Feb 2009, Max Laier wrote:
So to answer your first question, submitting 101 patches to rearrange 101
structs is certainly a wasted effort. However, if you take a good look at
the 2000 holes, identify an interesting subset and submit a patch to fix
that subset ... that would be a worthwhile effort ... IMHO.
The other thing to keep in mind is that structure layout can have a
noticeable effect on cache locality. Arbitrarily rearranging structure
members can generate many more cache misses so one should sanity check
changes w/ something like hwpmc. However as noted because layout may be
platform-dependent even if something shows no change on x86 it may be a
loss on another architecture and finding that performance drop may be
really hard.
Let's not be too "glass half empty" about it, though. The same is true in
the opposite direction. If we can identify and eliminate an unnecessary
hole in an important structure we might gain that same performance just by
reshuffling a few lines.
Well, I think we want to inform this through actual measurement. Right now,
tools like hwpmc track cache misses by point in executable code, but what
would be nice is if we could post-process to generate cache miss information
by data structure field...
Robert N M Watson
Computer Laboratory
University of Cambridge
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