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:
| % 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:
| % 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.
![[The future's so bright, I gotta wear shades]](https://www.conman.org/people/spc/about/2023/1023.t.jpg "The future's so bright, I gotta wear shades")