[RFC] Proposal to save a few hundreds bytes in the 'request' pool

Mon, 13 May 2013 14:05:33 -0700 

Hi,
trying to reduce httpd memory usage, I've worked on ap_rgetline_core in protocol.c. 

This function gets a line of protocol input.

To do so, :

    - it processes the apr_bucket_brigade it is given in parameter, one 
bucket at a time
    - for the common case, it allocates memory to store the data from 
the bucket, with a minimum on 80 bytes, to try to avoid other memory 
allocations, should there be other buckets
    - in case of several buckets and 80 bytes was not enough, it 
doubles the memory allocated and copy data in the new buffer



Why is this minimum of 80 bytes there ?

As stated in the comment, we assume the common case : only 1 bucket.

According to my testing, with a basic configuration and a simple 
request, this is the case.



So, I suggest,

    *  either to simply remove the test line 293 in protocol.c and the 
use of MIN_LINE_ALLOC

             if (current_alloc < MIN_LINE_ALLOC) {
                 current_alloc = MIN_LINE_ALLOC;
             }
       The already in place mechanism will grow the buffer if needed


    * or, if you prefer to over allocate in case of several buckets, 
turn it into:

             if (APR_BUCKET_NEXT(e) != APR_BRIGADE_SENTINEL(bb)) {
                if (current_alloc < MIN_LINE_ALLOC) {
                    current_alloc = MIN_LINE_ALLOC;
                }
             }
      to save memory when we know we can.



Personally, I prefer the first version and drop the test and the use of 
MIN_LINE_ALLOC.





Here is a example of the calls to ap_rgetline_core to process a request. 
In all cases, only 1 bucket was in the brigade.
If we allocated only what was needed, *416 bytes* would have been saved, 
which is, IMO, not to bad:


>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
---> ap_rgetline_core : 16 < 80 ? (only one bucket)
    --> 'GET / HTTP/1.1'

---> ap_rgetline_core : 17 < 80 ? (only one bucket)
    --> 'Host: localhost'

---> ap_rgetline_core : 88 < 80 ? No, allocate 88 (only one bucket)

    --> 'User-Agent: Mozilla/5.0 (X11; Ubuntu; Linux i686; rv:20.0) 
Gecko/20100101 Firefox/20.0'


---> ap_rgetline_core : 73 < 80 ? (only one bucket)

    --> 'Accept: 
text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8'


---> ap_rgetline_core : 54 < 80 ? (only one bucket)
    --> 'Accept-Language: fr,fr-fr;q=0.8,en-us;q=0.5,en;q=0.3'

---> ap_rgetline_core : 32 < 80 ? (only one bucket)
    --> 'Accept-Encoding: gzip, deflate'

---> ap_rgetline_core : 24 < 80 ? (only one bucket)
    --> 'Connection: keep-alive'

---> ap_rgetline_core : 18 < 80 ? (only one bucket)
    --> 'Pragma: no-cache'

---> ap_rgetline_core : 25 < 80 ? (only one bucket)
    --> 'Cache-Control: no-cache'

---> ap_rgetline_core : 2 < 80 ? (only one bucket)
    --> ''
>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>


Thanks for your comments.

best regards,
CJ























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