The Boston Diaries

The ongoing saga of a programmer who doesn't live in Boston, nor does he even like Boston, but yet named his weblog/journal “The Boston Diaries.”

Go figure.

Wednesday, October 24, 2007

Thoughts on optimizing a greylist daemon

Speaking of optimization, I thought it might be fun to profile the greylist daemon. It's not difficult—just recompile the program with the appropriate compiler option and run it.

Now, when I wrote the greylist daemon, I did gave thought to how I was going to search through the vast amount of information, and wrote the code with that in mind (oooh, premature optimizations? Or was that a mature optimization?). I was also curious as to any hotspots in the code, as I tend to max out at 130 requests per second on my development server.

The results were a bit surprising:

Each sample counts as 0.01 seconds.
% time cumulative seconds self seconds calls self Ts/call total Ts/calls name
% time cumulative seconds self seconds calls self Ts/call total Ts/calls name
50.00 0.01 0.01 661477 0.00 0.00 tuple_cmp_ift
50.00 0.02 0.01 1 10.00 10.00 whitelist_dump_stream
0.00 0.02 0.00 140892 0.00 0.00 edomain_cmp
0.00 0.02 0.00 108648 0.00 0.00 crc32
0.00 0.02 0.00 95853 0.00 0.00 edomain_search
0.00 0.02 0.00 28270 0.00 0.00 StreamEOF
0.00 0.02 0.00 27439 0.00 0.00 report_stderr
0.00 0.02 0.00 27273 0.00 0.00 ipv4
0.00 0.02 0.00 27165 0.00 0.00 check_signals
0.00 0.02 0.00 27162 0.00 0.00 send_packet
0.00 0.02 0.00 27155 0.00 0.00 ip_match
0.00 0.02 0.00 27155 0.00 0.00 send_reply
0.00 0.02 0.00 27155 0.00 0.00 type_graylist
0.00 0.02 0.00 25458 0.00 0.00 tuple_search
0.00 0.02 0.00 24692 0.00 0.00 tuple_add
0.00 0.02 0.00 24692 0.00 0.00 tuple_allocate

The big surprise was that the execution time was split between two functions, tuple_cmp_ift() (which compares two tuples) and whitelist_dump_stream(), and the amusing bit is the disparity of the number of calls between the two, 661,477 calls to the former vs. the one call to the latter.

But in this case, the one call to whitelist_dump_stream() was made when the program ended (all it does is write out the tuples that have been whitelisted). Remove that from the program, and we pretty much have our hotspot—tuple_cmp_ift().

The other surprise is that there weren't any real surprises. The test data consists of 27,155 tuples with some duplicates, and you can see the various checks were called that many times. The only reason edomain_search() was called as many times as it was is that there are four lists that are checked 24,000 times each. crc32 is called twice for each packet (yes, that's intentional) so divide that by four (request and response) and it fits.

The code is pretty much even except for tuple_cmp_ift(), which is our obvious hotspot.

int tuple_cmp_ift(const void *left,const void *right)
{
  const struct tuple *l = left;
  const struct tuple *r = right;
  int                 rc;

  ddt(left   != NULL);          /* similar to assert() */
  ddt(right  != NULL);          /* but logs to syslogd */
  ddt(l->pad == 0xDECAFBAD); /* compiled out of profiled */
  ddt(r->pad == 0xDECAFBAD); /* code */

  /*-------------------------------
  ; sizeof(l->ip) is 16 bytes, enough
  ; space to hold an IPv6 address; not 
  ; that we use IPv6 currently, but planning
  ; for the future.
  ;------------------------------------------*/

  if ((rc = memcmp(l->ip,r->ip,sizeof(l->ip))) != 0) return(rc);

  if (l->fromsize < r->fromsize)
    return(-1);
  else if (l->fromsize > r->fromsize)
    return(1);

  if ((rc = memcmp(l->from,r->from,l->fromsize)) != 0) return(rc);

  if (l->tosize < r->tosize)
    return(-1);
  else if (l->tosize > r->tosize)
    return(1);

  rc = memcmp(l->to,r->to,l->tosize);
  return(rc);
}

Now, before I spend any time trying to optimize this bit of code, I thought I'd rerun the program without it. I changed tuple_search() (which calls tuple_cmp_ift()) to simply return “not found,” and I removed the call to whitelist_dump_stream(). The results were even more amusing:

Each sample counts as 0.01 seconds.
no time accumulated
% time cumulative seconds self seconds calls self Ts/call total Ts/calls name
% time cumulative seconds self seconds calls self Ts/call total Ts/calls name
0.00 0.00 0.00 140892 0.00 0.00 edomain_cmp
0.00 0.00 0.00 108668 0.00 0.00 crc32
0.00 0.00 0.00 95853 0.00 0.00 edomain_search
0.00 0.00 0.00 27438 0.00 0.00 report_stderr
0.00 0.00 0.00 27273 0.00 0.00 ipv4
0.00 0.00 0.00 27169 0.00 0.00 check_signals
0.00 0.00 0.00 27167 0.00 0.00 send_packet
0.00 0.00 0.00 27155 0.00 0.00 ip_match
0.00 0.00 0.00 27155 0.00 0.00 send_reply
0.00 0.00 0.00 27155 0.00 0.00 type_graylist
0.00 0.00 0.00 25458 0.00 0.00 tuple_add
0.00 0.00 0.00 25458 0.00 0.00 tuple_allocate
0.00 0.00 0.00 25458 0.00 0.00 tuple_search
0.00 0.00 0.00 3577 0.00 0.00 StreamEOF

Yeah, a lot of called functions, but not enough accumulated time to even survive rounding up.

I'm begining to think that the 130 requests per second limit I'm seeing isn't a function of the code, but of the network stack. Given this result, I doubt I'll bother optimizing tuple_cmp_ift() any time soon, which seems to re-enforce the whole “premature optimization is eeeeevil” thought, but in reality, I wrote the code around a few optimizations (namely, well considered data structures) in the beginning and didn't need to optimize it later.

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