doc/config: user documentation for limiters

I've relied on this feature to keep the VPS behind
https://public-inbox.org/git/ from OOM-ing since 2016,
so document it to ensure others can make use of low-end
servers like I do.

More limiters may become configurable for viewvcs and
solver functionality (or we continue using the default
one).

diff --git a/Documentation/public-inbox-config.pod b/Documentation/public-inbox-config.pod
index 23ebcc5fc6c1655e478ba8800f314a4f32741c99..cad0c4ee2376d3d8181fa792440c0a093c12576a 100644 (file)
--- a/Documentation/public-inbox-config.pod
+++ b/Documentation/public-inbox-config.pod
@@ -91,6 +91,18 @@ C<nntp://news.gmane.org/gmane.mail.public-inbox.general>
 
 Default: none
 
+=item publicinbox.<name>.httpbackendmax
+
+If a digit, the maximum number of parallel
+L<git-http-backend(1)> processes to allow for cloning this
+particular inbox.
+
+If an alphanumeric value starting with a lowercase alphabetic
+character is specified, the inbox will use a L</NAMED LIMITER>
+which can be shared by multiple inboxes.
+
+Default: 32 (using a default limiter shared by all inboxes)
+
 =item publicinbox.<name>.coderepo
 
 The nickname of a "coderepo" section associated with the inbox.
@@ -169,6 +181,53 @@ Default: none
 
 =back
 
+=head2 NAMED LIMITER (PSGI)
+
+Named limiters are useful for preventing large inboxes from
+monopolizing (or overloading) the server.  Since serving git
+clones (via L<git-http-backend(1)> can be memory-intensive for
+large inboxes, it makes sense to put large inboxes on a named
+limiter with a low max value; while smaller inboxes can use
+the default limiter.
+
+=over 8
+
+=item publicinboxlimiter.<name>.max
+
+The maximum number of parallel processes for the given limiter.
+
+=back
+
+=head3 EXAMPLE WITH NAMED LIMITERS
+
+       ; big inboxes which require lots of memory to clone:
+       [publicinbox "big1"]
+               mainrepo = /path/to/big1
+               address = big1@example.com
+               httpbackendmax = big
+       [publicinbox "big2"]
+               mainrepo = /path/to/big2
+               address = big2@example.com
+               httpbackendmax = big
+
+       ; tiny inboxes which are easily cloned:
+       [publicinbox "tiny1"]
+               mainrepo = /path/to/tiny1
+               address = tiny1@example.com
+       [publicinbox "tiny2"]
+               mainrepo = /path/to/tiny2
+               address = tiny2@example.com
+
+       [publicinboxlimiter "big"]
+               max = 4
+
+In the above example, the "big1" and "big2" are limited to four
+parallel L<git-http-backend(1)> processes between them.
+
+However, "tiny1" and "tiny2" will share the default limiter
+which means there can be 32 L<git-http-backend(1)> processes
+between them.
+
 =head1 ENVIRONMENT
 
 =over 8