On May 11, 2009, at 11:46 AM, Francesco Riosa wrote:
On Monday 11 May 2009 17:53:59 Tim Soderstrom wrote:
On May 11, 2009, at 10:47 AM, Francesco Riosa wrote:
On Monday 11 May 2009 16:53:23 Jay Pipes wrote:
Francesco Riosa wrote:
gcc -c -Q -O3 --help=optimizers | grep funroll-loops
-funroll-loops [disabled]
-funroll-loops is the first branch in the root of evil, don't rely
on it,
even force -O3 isn't as good as it seem often.
Hehe, yes, I'm aware of the dangers :) /me is not a Gentoo user ;)
No comment, you little anti-gentoo fanboy :)
Actually, what about -Os (without -funroll-loops)? -Os does more
optimizations than -O2 but fewer than -O3 in an effort to produce a
smaller foot-print. I have not done an extensive test, but did notice
that it made noticeably smaller binaries when I used it.
Actually it does some less:
-falign-jumps
-falign-labels
-falign-loops
-finline-functions
-freorder-blocks
-ftree-ch
-ftree-pre
-Os is -O2 minus code growing optimizations
I did use -Os with mysql 5.1 happily and with a rather noticeable
slimmer
mysqld, expecially on 64 bit (yes I'm a bit richer too)
Right so the question is - what is more important to cache in L1/L2/L3
- Drizzle or the data it is operating on? I'm thinking the latter.
Does unrolling loops really help all that much if the data we need to
work on isn't in the cache? If the code footprint is smaller, that
means we can put more data into cache. Besides, it's harder to control
the caching of data (I would think) than controlling the code foot-
print.
Practically, I haven't looked near that deep when it comes to DB
performance so I'm mostly just commenting for the safe of
conversation :)
I agree, however, for things other than desktop applications that
don't do a lot of context-switching, -funroll-loops is terrible
because it makes the program size so much bigger, which tends to eat
up a lot of cache. Again, I haven't do an extensive test, but that's
what I found out on a high level when I played around with it.
http://funroll-loops.info/
already in my bookmarks ... since 2005 ;)
I loved that site even when I was a Gentoo user. I refuse to call
myself a ricer because the only optimizations I used were -O3 and
later -Os. Well, that and -fomit-frame-pointer occasionally :)
Tim S.
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