Competing for speed

Article: Bulldozer Benchmarks, What Do They Mean?
By: ? (0xe2.0x9a.0x9b.delete@this.gmail.com), April 18, 2011 11:45 pm
Room: Moderated Discussions
Linus Torvalds (torvalds@linux-foundation.org) on 4/18/11 wrote:
---------------------------
>? (0xe2.0x9a.0x9b@gmail.com) on 4/18/11 wrote:
>>
>>I am failing to find the words. According to you, the fact that you are *not being
>>smart* about avoiding work in the initial phases has no relevance to the *relative*
>>amount of time that SParse spends in the later phases. Is that supposed to be some kind of joke?
>
>You do seem to have some issues, atom.

Of course I do.

>The issue really is that you cannot avoid the initial
>phases like lexing and parsing. They need to do the
>whole thing.

Well, I never wrote anything which would state the opposite (i.e: I never wrote that if you have a C source-code file with unknown contents then it is possible to ignore some of the characters the file contains).

Which part of the sentence "to optimize across multiple executions of SParse" you do not understand?

>The later phases can avoid the irrelevant
>stuff, and not even see them (and I'm not just talking
>the trivial case of comments etc, but inline functions
>that were never used in anything that had to had its code
>generated etc).
>
>I bet a lot of compilers end up doing a fair amount of
>work on generating some fairly complicated internal
>representation of inline functions and structure definitions
>whether they are used or not.
>
>Sparse doesn't. Sparse only creates the parse tree, and
>doesn't do anything complicated with it (until needed).
>
>But note that it _does_ do the actual parse tree for it
>all. So when I say it's more "front-end heavy" than it
>might otherwise be, that front-end does still include the
>parser.
>
>And despite that, the lexical part is still more expensive
>than the parsing part.
>
>>The reason why SParse is implemented the way it is implemented lies in the fact
>>that you never competed for speed. You stopped optimizing it when the speed was "good enough".
>
>You've now repeated that complete red herring twice, with
>nothing to back it up.

Of course. So which part of the sentence "to optimize across multiple executions of SParse" you do not understand?

>The front-end is optimized too.
>
>Is it "optimized to some theoretical fastest possible
>case"? No. But it's not bad, and I spent some time with
>a profiler with it (it's been years since that, so..)

The fact that you simply ignored an optimization opportunity to make SParse N-times faster may imply that a non-trivial portion of the time you spent with the profiler was wasted. I repeat: wasted.

>In fact, I think it's optimized a lot more than the
>parser, exactly because it _did_ show up in profiles.
>
>For example, you pointed to already_tokenized() using a
>simple linear lookup as some reason why it's not optimal,

The linear search is not the only reason why it isn't optimal. The strings you are comparing there have a common prefix (e.g: "include/asm-generic/...", "include/linux/...").

>but the _big_ optimization there is to not even tokenize
>the common kind of
>
>#ifndef _HEADERNAME_H
>#define _HEADERNAME_H
>.. header file contents here ..
>#endif
>
>files twice. The linear loop over the currently included
>files was always so small that it didn't matter in
>practice that we loop over the list stupidly.

So, 5% is "small". If 5% is small, then what is the threshold for you to even consider to optimize a function? 90%?

>Could things be improved? Yes. If you want to optimize it
>further, go right ahead - I'll take patches (or rather,
>I'll send them on). But the tokenization code is by no
>means horrible.

The main point was not about SParse, but about the fact that you were making generalizations to software other than SParse.

>(One thing that makes tokenization slightly slower is that
>the position information is packed into bitfields, because
>it turns out that memory use was a fairly big issue for
>performance - cache misses cost more than just about
>anything. So look at "struct position", and all the unholy
>code generated to fill in that at tokenization time).

In the test I did on my computer, I can see that LLC (last-level cache) misses are totally insignificant. So, my data contradicts what you just wrote.

How do you explain that? (... and don't forget to dodge this question like Neo dodging bullets.)

>And the real point was that parsing is fast and simple. It
>was never the problem.

So, here we have the general claim once again I suppose.

I really don't understand how you can make the *general* claim that "parsing is fast in comparison to other stuff in the front-end" if it is clear that *your* front-end is suboptimal.

Why don't you simply admit that you are *not entitled* to make general claims about such things ???
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                                Imperative compilers (and the lack of it)none2011/04/10 06:00 AM
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