How do I get SpamAssassin to run faster?

General Advice

Examine the custom rule set files you use:

• Avoid large rule sets, those over 100k or 150k in size. The more rules you have, the slower SA will run.
• In particular, the blacklist and blacklist-uri (aka sa-blacklist and sa-blacklist-uri) rulesets are extremely heavyweight, and massively affect performance and memory usage. If you're using these, remove them immediately. Ensure network tests are enabled, instead; those rules have been made obsolete by the URIBL_WS_SURBL network rule. See OutOfMemoryProblems.
• Pick rule set files that are more productive. In the SARE families published by Bob Menschel, use files 0 and 1 for productivity / efficiency, and avoid files 2 and 3.
• Remove and re-add rule set files one at a time, and check performance after each change. If one rule set file causes a huge change in performance, take appropriate action.
  

Examine the custom rules you create, or have downloaded from third parties. Poorly-written regular expressions can use resources exponentially. Avoid body, rawbody, or full rules that use + or * quantifiers.

Use spamd.

Use sa-compile if you're running SA 3.2.x or later.

If you are using network tests, install a local DNS server (BIND named, for example) on the same host to cache responses, and set the /etc/resolv.conf file to use that instead of one on another machine. See CachingNameserver.

If you're seeing periodic load spikes, it could be because the Bayes database is re-syncing and expiring old tokens. See BayesForceExpire for a workaround.

If you are memory-bound

If the spamd processes are eating up all the RAM on your machine, then you are memory-bound.

Are you experiencing high system load or possibly swapping? Look at the number of children you have spawned, and compare that to the available memory (by default each child can use 20-30 megs of RAM). Depending on load you might find success in lowering the number of children that are spawned (see -m in the spamd documentation).

If you are running with network tests active, you may be able to reduce memory load by turning off some of them. See "Network related" below.

See also OutOfMemoryProblems.

If you are I/O-bound

For heavily loaded servers, you may be experiencing high iowait times depending on how hard you are hitting your disk. You can try offloading the logging and bayes disk writes to a separate disk, or even disabling Bayes rules entirely with use_bayes 0.

If the auto-whitelist is in use (user config dirs contain files named "auto-whitelist"), you should turn that off; it provides a marginal gain in accuracy for quite a bit of I/O load. Set use_auto_whitelist 0.

If you are CPU-bound

If your server is being limited by CPU load:

Use sa-compile.

Remove custom rule sets, as detailed above. Seriously.

See OutOfMemoryProblems. Much of the advice applies for CPU-bound machines, too.

Network related

If you are UsingNetworkTests, install a local DNS server (BIND named, for example) on the same host to cache responses, and set the /etc/resolv.conf file to use that instead of one on another machine. See CachingNameserver.

External network tests often take long time (compared to a non-network installation). See UsingNetworkTests for general hints.

• Pyzor (see UsingPyzor) tests seem to be one of the most time consuming: it may raise scanning time by more than 10 seconds per check.
• DCC (see UsingDcc) may add 3-5 seconds to testing time.
  

Consider turning off network tests, and running with "-L", if you can afford a large drop in accuracy. This is not a very good option for most people though, and while it will reduce system memory load by reducing the number of simultaneous processes, it will increase system CPU load, so be warned! See NetworkTestsLatency for more info.

Finding slow rules

Save http://svn.apache.org/repos/asf/spamassassin/trunk/masses/plugins/HitFreqsRuleTiming.pm to ~/.spamassassin/

In ~/.spamassassin/user_prefs add a line containing:
loadplugin HitFreqsRuleTiming HitFreqsRuleTiming.pm

Run spamassassin on an email.

Find timing.log in your current directory, containing the number of seconds each rule took to run, in the 2nd and 3rd column.

Warning: Enabling this will almost certainly slow down SpamAssassin. Possibly a lot (I haven't checked).