doc: strongly recommend MALLOC_MMAP_THRESHOLD_=131072 for glibc HEAD master

The 131072 byte lower bound was the old default before the
sliding mmap window was introduced in modern glibc malloc.
While the sliding mmap window was intended to be faster by
reducing syscalls, zeroing and kernel overhead, it is also prone
to fragmentation from allocation patterns seen in evented Perl
servers.

Individual allocations over 128K are rare in our codebase since
there aren't many messages this large, making any performance
impact tiny.  Furthermore, the reduction in fragmentation and
memory use will be a speedup for memory-constrained systems
since they can avoid swap and have more leftover for the page
cache.

6 files changed, 18 insertions, 12 deletions

diff --git a/Documentation/RelNotes/v2.0.0.wip b/Documentation/RelNotes/v2.0.0.wip
index 4d872fd7..794d7956 100644
--- a/Documentation/RelNotes/v2.0.0.wip
+++ b/Documentation/RelNotes/v2.0.0.wip
@@ -54,8 +54,9 @@ treewide
 
   * SHA-256 coderepos are fully supported (but not inboxes, yet)
 
-  * jemalloc (tested as an LD_PRELOAD) is recommended to reduce fragmentation
-    in long-running daemon processes serving unpredictable traffic
+  * for daemons serving public traffic, MALLOC_MMAP_THRESHOLD_=131072 is
+    recommended to reduce fragmentation in glibc malloc, while jemalloc
+    (tested as an LD_PRELOAD) is another option.
 
 PublicInbox::WWW
 
diff --git a/Documentation/public-inbox-tuning.pod b/Documentation/public-inbox-tuning.pod
index 7d0690b4..892ee0f2 100644
--- a/Documentation/public-inbox-tuning.pod
+++ b/Documentation/public-inbox-tuning.pod
@@ -165,11 +165,10 @@ capacity planning.
 
 Bursts of small object allocations late in process life contribute to
 fragmentation of the heap due to arenas (slabs) used internally by Perl.
-jemalloc (tested as an LD_PRELOAD on GNU/Linux) reduces
-overall fragmentation compared to glibc malloc in long-lived processes.
-glibc malloc users may try setting C<MALLOC_MMAP_THRESHOLD_> to a lower
-value (e.g. 131072) but that may require increasing the
-C<sys.vm.max_map_count> sysctl.
+glibc malloc users should use C<MALLOC_MMAP_THRESHOLD_=131072> to reduce
+fragmentation from the sliding mmap window.  jemalloc (tested as an
+LD_PRELOAD on GNU/Linux) also reduces fragmentation compared to an
+unconfigured glibc malloc in long-lived processes.
 
 =head2 Other OS tuning knobs
 
diff --git a/examples/public-inbox-httpd@.service b/examples/public-inbox-httpd@.service
index 11859198..ca68fc7e 100644
--- a/examples/public-inbox-httpd@.service
+++ b/examples/public-inbox-httpd@.service
@@ -19,6 +19,7 @@ After = public-inbox-httpd.socket
 Environment = PI_CONFIG=/home/pi/.public-inbox/config \
 PATH=/usr/local/bin:/usr/bin:/bin \
 TZ=UTC \
+MALLOC_MMAP_THRESHOLD_=131072 \
 PERL_INLINE_DIRECTORY=/tmp/.pub-inline
 
 LimitNOFILE = 30000
diff --git a/examples/public-inbox-imapd@.service b/examples/public-inbox-imapd@.service
index 80104605..1aede65d 100644
--- a/examples/public-inbox-imapd@.service
+++ b/examples/public-inbox-imapd@.service
@@ -16,6 +16,8 @@ After = public-inbox-imapd.socket
 [Service]
 Environment = PI_CONFIG=/home/pi/.public-inbox/config \
 PATH=/usr/local/bin:/usr/bin:/bin \
+TZ=UTC \
+MALLOC_MMAP_THRESHOLD_=131072 \
 PERL_INLINE_DIRECTORY=/tmp/.pub-inline
 
 LimitNOFILE = 30000
diff --git a/examples/public-inbox-netd@.service b/examples/public-inbox-netd@.service
index 7437f086..51f58fbb 100644
--- a/examples/public-inbox-netd@.service
+++ b/examples/public-inbox-netd@.service
@@ -12,14 +12,15 @@ Wants = public-inbox-netd.socket
 After = public-inbox-netd.socket
 
 [Service]
-# An LD_PRELOAD for libjemalloc can be added here.  It is
-# more resistant to fragmentation in long-lived daemons than glibc.
-# If you're unable to use jemalloc, setting MALLOC_MMAP_THRESHOLD_
-# to a lower value (e.g. 131072) but that may also require increasing
-# the sys.vm.max_map_count sysctl.
+
+# Setting MALLOC_MMAP_THRESHOLD_=131072 reduces fragmentation by
+# disabling the sliding mmap window in glibc malloc.  An LD_PRELOAD for
+# libjemalloc may be added here, instead.  jemalloc is more resistant to
+# fragmentation in long-lived daemons than unconfigured glibc malloc.
 Environment = PI_CONFIG=/home/pi/.public-inbox/config \
 PATH=/usr/local/bin:/usr/bin:/bin \
 TZ=UTC \
+MALLOC_MMAP_THRESHOLD_=131072 \
 PERL_INLINE_DIRECTORY=/tmp/.netd-inline
 
 LimitNOFILE = 30000
diff --git a/examples/public-inbox-nntpd@.service b/examples/public-inbox-nntpd@.service
index 24f9ca73..556cb76f 100644
--- a/examples/public-inbox-nntpd@.service
+++ b/examples/public-inbox-nntpd@.service
@@ -16,6 +16,8 @@ After = public-inbox-nntpd.socket
 [Service]
 Environment = PI_CONFIG=/home/pi/.public-inbox/config \
 PATH=/usr/local/bin:/usr/bin:/bin \
+TZ=UTC \
+MALLOC_MMAP_THRESHOLD_=131072 \
 PERL_INLINE_DIRECTORY=/tmp/.pub-inline
 
 LimitNOFILE = 30000