doc: tuning: add note about 64-bit OpenSSL speedup

Since switching HTTPS termination to 64-bit, I've noticed a
significant CPU usage reduction on public-inbox.org.  IMAPS,
NNTPS, and POP3S remain 32-bit for the moment since that doesn't
go through varnish and I haven't gotten around to dealing with
a 64-bit Xapian (or SQLite) install.

My current HTTPS setup is:

  64-bit PublicInbox::PsgiRproxy -> varnish -> 32-bit PublicInbox::WWW

IMAPS, NNTPS, POP3S hit the same 32-bit public-inbox-netd
process directly without a caching layer.

diff --git a/Documentation/public-inbox-tuning.pod b/Documentation/public-inbox-tuning.pod
index 3f2c086164ee1e14628d08dc906043f6b3e58604..36f17b36f148bba277de7046300009ffb6c7a465 100644 (file)
--- a/Documentation/public-inbox-tuning.pod
+++ b/Documentation/public-inbox-tuning.pod
@@ -159,9 +159,10 @@ The open file descriptor limit (C<RLIMIT_NOFILE>, C<ulimit -n> in L<sh(1)>,
 C<LimitNOFILE=> in L<systemd.exec(5)>) may need to be raised to
 accommodate many concurrent clients.
 
-Transport Layer Security (IMAPS, NNTPS, or via STARTTLS) significantly
-increases memory use of client sockets, be sure to account for that in
-capacity planning.
+Transport Layer Security (HTTPS, IMAPS, NNTPS, or via STARTTLS)
+significantly increases memory use of client sockets, be sure to account
+for that in capacity planning.  Switching L<https://public-inbox.org/>
+to a 64-bit userspace reduced libcrypto.so (OpenSSL) CPU use by ~70%.
 
 Bursts of small object allocations late in process life contribute to
 fragmentation of the heap due to arenas (slabs) used internally by Perl.