From 8fd4e5f47efd13a6bad7811239438539ae383966 Mon Sep 17 00:00:00 2001
From: Anders Svensson <anders@erlang.org>
Date: Mon, 7 Dec 2015 17:06:38 +0100
Subject: Make peer handling more efficient

Each service process maintains a dictionary of peers, mapping an
application alias to a {pid(), #diameter_caps{}} list of connected
peers. These lists are potentially large, peers were appended to the end
of the list for no particular reason, and these long lists were
constructed/deconstructed when filtering them for pick_peer callbacks.
Many simultaneous outgoing request could then slow the VM to a crawl,
with many scheduled processes mired in list manipulation.

The pseudo-dicts are now replaced by plain ets tables. The reason for
them was (once upon a time) to have an interface interchangeable with a
plain dict for debugging purposes, but strict swapablity hasn't been the
case for some time now, and in practice a swap has never taken place.

Additional tables mapping Origin-Host/Realm have also been introduced,
to minimize the size of the peers lists when peers are filtered on
host/realm. For example, a filter like

  {any, [{all, [realm, host]}, realm]}

is probably a very common case: preferring a Destination-Realm/Host
match before falling back on Destination-Realm alone. This is now more
efficiently (but not equivalently) expressed as

  {first, [{all, [realm, host]}, realm]}

to stop the search when the best match is made, and extracts peers from
host/realm tables instead of searching through the list of all peers
supporting the application in question. The code to try and start with a
lookup isn't exhaustive, and the 'any' filter is still as inefficient as
previously.
---
 lib/diameter/doc/src/diameter.xml | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

(limited to 'lib/diameter/doc/src')

diff --git a/lib/diameter/doc/src/diameter.xml b/lib/diameter/doc/src/diameter.xml
index b0cff32c9a..dbbbb01138 100644
--- a/lib/diameter/doc/src/diameter.xml
+++ b/lib/diameter/doc/src/diameter.xml
@@ -466,7 +466,7 @@ Matches only those peers whose Origin-Host has the
 specified value, or all peers if the atom <c>any</c>.</p>
 </item>
 
-<tag><c>{realm, any|&dict_DiameterIdentity;</c></tag>
+<tag><c>{realm, any|&dict_DiameterIdentity;}</c></tag>
 <item>
 <p>
 Matches only those peers whose Origin-Realm has the
@@ -499,18 +499,22 @@ Matches only those peers matched by each filter in the specified list.</p>
 <item>
 <p>
 Matches only those peers matched by at least one filter in the
-specified list.</p>
+specified list.
+The resulting list will be in match order, peers matching the
+first filter of the list sorting before those matched by the second,
+and so on.</p>
+</item>
 
+<tag><c>{first, [&peer_filter;]}</c></tag>
+<item>
 <p>
-The resulting peer list will be in match order, peers matching the
-first filter of the list sorting before those matched by the second,
-and so on.
-For example, the following filter causes peers matching both the host
-and realm filters to be presented before those matching only the realm
-filter.</p>
+Like <c>any</c>, but stops at the first filter for which there are
+matches, which can be much more efficient when there are many peers.
+For example, the following filter causes only peers best matching
+both the host and realm filters to be presented.</p>
 
 <pre>
-{any, [{all, [host, realm]}, realm]}
+{first, [{all, [host, realm]}, realm]}
 </pre>
 </item>
 
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