[cfe-dev] clang performance when building Linux

[Chandler Carruth]

On Sat, Apr 16, 2011 at 1:48 PM, Török Edwin <edwintorok at gmail.com> wrote:

> On 2011-04-16 23:32, Chandler Carruth wrote:
> > On Sat, Apr 16, 2011 at 5:01 AM, Benjamin Kramer
> > <benny.kra at googlemail.com <mailto:benny.kra at googlemail.com>> wrote:
> >
> >     >     3.65%     clang  clang                              [.]
> >     llvm::StringMapImpl::LookupBucketFor(llvm::StringRef)
> >
> >     I'm a bit surprised that StringMap is the most expensive entry here,
> >     maybe microoptimizing
> >     the hash function (which is a byte-wise djb hash at the moment) can
> >     help a bit. If someone is
> >     really bored it would also be useful to test if other string hash
> >     functions like murmurhash or google's
> >     new city hash give better performance.
> >
> >
> > Interesting. I'm familiar with murmurhash and watched the development of
> > city hash and am quite familiar with it. I'll take a look at what it
> > would take to use cityhash here. Anything special done to produce these
> > numbers? Just a build of the kernel?
> >
> > If you could paste how you collected the perf data that would be useful
> > as well... i've not used the 'perf' tool extensively before.
>
> Here is what I used:
> $ make allmodconfig
> $ perf record make CC=clang -j6
> (this creates a file perf.data, let it run for at least 2 or 5 minutes,
> then interrupt it, or wait for it to finish)
> $ perf report
> (ncurses-like interface to browse perf.data)
>

Cool, thanks!

I was never able to get the lookup to take as much of my CPU time as you
did, but the benchmarks were very noisy. When I used my own stress test
benchmarks (massive C++ file and the single-source GCC file) I would see
roughly 1.5% of the CPU cycles in this function.

I got CityHash into the codebase and taught StringMap to use it. This saved
roughly 50% of the time in the function, taking it under the 1% line. I
haven't looked in detail to see what is taking the time now.

On another benchmark where this function was a bit hotter (2.4% roughly,
similar numbers to those I got by profiling the kernel build) I saw as much
as 1% over-all speed up. Nothing stellar, but not terrible either.

If folks are interested, I'll look at getting City Hash checked in, and
investigate using it in a few other places as well where collisions and/or
hashing cost us some.
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