[asterisk-dev] Wanted: Missing CPU Cycles... Reward!

[Vadim Lebedev]

Steve Murphy wrote:
> On Wed, 2007-10-31 at 18:46 -0600, Steve Murphy wrote:
>   
>> On Wed, 2007-10-31 at 18:05 +0100, Vadim Lebedev wrote:
>>     
>>> ...
>>> Well the following is obviously MUUCH faster than the original:
>>>
>>> static void pbx_substitute_variables(char *passdata, int datalen,
>>> struct ast_channel *c, struct ast_exten *e)
>>> {
>>>
>>>         /* No variables or expressions in e->data, so why scan it?
>>> */
>>>         if (e->data && !strchr(e->data, '$'))
>>>                 ast_copy_string(passdata, e->data, datalen);
>>>                 return;
>>>         }
>>>         
>>>         *passdata = 0;
>>>         pbx_substitute_variables_helper(c, e->data, passdata,
>>> datalen - 1);
>>> }
>>>
>>>
>>> Original:
>>> static void pbx_substitute_variables(char *passdata, int datalen,
>>> struct ast_channel *c, struct ast_exten *e)
>>> {
>>>         memset(passdata, 0, datalen);
>>>
>>>         /* No variables or expressions in e->data, so why scan it?
>>> */
>>>         if (e->data && !strchr(e->data, '$') && !
>>> strstr(e->data,"${") && !strstr(e->data,"$[") && !
>>> strstr(e->data,"$(")) {
>>>                 ast_copy_string(passdata, e->data, datalen);
>>>                 return;
>>>         }
>>>
>>>         pbx_substitute_variables_helper(c, e->data, passdata,
>>> datalen - 1);
>>> }
>>>
>>>
>>>       
>> Oh, and yes, I forgot to mention *passdata = 0; vs memset(passdata, 0,
>> datalen);
>> Considering that the passdata buffer is 8192 bytes long, just writing
>> 1 byte vs. 8192 bytes WOULD be a huge savings in time. 
>>
>> BUT, you can't do that. pbx_substitute_variables_helper will not work.
>> It depends on passdata being nulled out, and it will fail to work
>> correctly if you go that route.
>>
>> murf
>>     
>
> Hmmmm. I wrote another letter, that seems not to have made it, saying
> that the part about not unnecessarily scanning the e->data is a good
> idea, but my benchmark, there's always a ${} or $[] expression, or both,
> so such an improvement won't help... and, that Tilghman has just added
> that improvement to trunk... (a good idea is a good idea!)
>
> But your *passdata = 0; idea got me to thinking. I determined to
> re-write pbx_substitute_variables_helper_full() to eliminate the need to
> zero out that 8K buffer 4 million times (for 4 million priorities to
> execute). The further I studied how
> pbx_substitute_variables_helper_full() was built, tho, the more I liked
> it. So I gave up the idea of rewriting it, to just tweaking it to zero
> out the byte after the last byte copied to the output buffer. That
> turned out to be pretty simple. There are only 3 places in the routine
> where bytes are copied.
>
> That done, there were 2 places, where a 4K buffer was memset to zeroes,
> for a recursive call. I got rid of these, also. Then, I went thru all
> the source, and got rid of all the memsets (and array initializations)
> in all the places it was done, to zero the buffer for
> pbx_substitute_variables_helper/_full, and ran my perf measurements
> again. Now, without having to zero out between 32 and 48 Gbytes of
> buffer, it's now at 100K!!!
>
> See http://svn.digium.com/svn/asterisk/team/murf/fast-ast3 for all this
> work. If everybody would please review the mods, and test it, I'd
> appreciate it. I'd like to merge this into trunk, as soon as possible,
> if everyone is willing/no-one objects.
>
> murf
>
>   
Steve, you was faster than me... :(
I was doing EXACTLY the same thing....

Congrats


Vadim


P.S. Another way to increase substitution perfomance is on first call 
'compile' it into a primitive  'machine'
code for a substitution machine. So on subsequent calls the parsing will 
be skipped alltogether.
I'd exepect 10 fold perfomance increase this way.
But it will take much more time to develop

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