doc: tuning: add a note about Linux sys.vm.max_map_count

git tends to die when mmap(2) fails on this limit, so let
users know about it.  Perhaps git could gracefully fallback.

diff --git a/Documentation/public-inbox-tuning.pod b/Documentation/public-inbox-tuning.pod
index 58937f732dd157dd6be993ba01e7172337131041..67316fb11109b4071197a2db9788dc254eb9a1d0 100644 (file)
--- a/Documentation/public-inbox-tuning.pod
+++ b/Documentation/public-inbox-tuning.pod
@@ -34,6 +34,10 @@ Performance on solid state drives
 
 Read-only daemons
 
+=item 7
+
+Other OS tuning knobs
+
 =back
 
 =head2 New inboxes: public-inbox-init -V2
@@ -148,6 +152,15 @@ Transport Layer Security (IMAPS, NNTPS, or via STARTTLS) significantly
 increases memory use of client sockets, sure to account for that in
 capacity planning.
 
+=head2 Other OS tuning knobs
+
+Linux users: the C<sys.vm.max_map_count> sysctl may need to be increased if
+handling thousands of inboxes (with L<public-inbox-extindex(1)>) to avoid
+out-of-memory errors from git.
+
+Other OSes may have similar tuning knobs (patches appreciated).
+
+
 =head1 CONTACT
 
 Feedback encouraged via plain-text mail to L<mailto:meta@public-inbox.org>