This is the mail archive of the mailing list for the GCC project.

Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]
Other format: [Raw text]

Re: Faster compilation speed

On Tue, 20 Aug 2002, Richard Earnshaw wrote:
> > I had done that on alpha, but didn't initially report the figures.  Would
> > a comparison to 2.95 also be useful?
> Certainly -- the numbers don't really mean anything unless we have
> something to compare them against.

I figured so.  (Wow, I hadn't built a 2.95 toolchain in a long time.)

> > gcc version 3.3 20020802 (experimental)
> >
> > ---------------------------------------------------------------------------
> > cc1 -O2 reload.i
> >
> > issues/cycles = 0.51  issues/dcache_miss = 26.93  issues/dtb_miss = 1214.36

gcc version 2.95.3 20010315 (release)

cc1 -O2 reload.i
issues/cycles = 0.54  issues/dcache_miss = 26.31  issues/dtb_miss = 2488.

cc1 reload.i
issues/cycles = 0.52  issues/dcache_miss = 26.30  issues/dtb_miss = 3306.

Now that's interesting.  No real change in L1 cache performance, but TLB
misses nearly cut in half vs. 3.3.

Trying L3 misses (both with -O0):

3.3: issues/bcache_miss = 370
2.95.3: issues/bcache_miss = 437

Wall-clock time is nearly 2/1 for these tests, as are TLB misses, while
other stats are close.  Hmm.

> So if I understand these figures correctly, then
> dcache_miss/dtb_miss ~= 45
> That is, one in 45 dcache fetches also requires a tlb walk.

That's how I see it.

> How many dtb entries does an Alpha have?

No idea.  This is an ev56.  I could try grabbing the specs from Digital's
site, if I can still find it...

How expensive is a TLB miss, anyway?  I hadn't expected it would occur
often enough in gcc to be significant.  Note the IPC ratio stays constant,
but as I understand it, TLB is handled in software, so maybe those cycles
are counted by iprobe?


Index Nav: [Date Index] [Subject Index] [Author Index] [Thread Index]
Message Nav: [Date Prev] [Date Next] [Thread Prev] [Thread Next]