8006742: Initial TLAB sizing heuristics might provoke premature GCs

roshan mangal roshanmangal at gmail.com
Wed Jun 13 11:23:43 UTC 2018

Hi Everyone,

This is my first patch as a new member of OpenJDK community.

I have looked into minor bug
https://bugs.openjdk.java.net/browse/JDK-8006742 ( Initial TLAB sizing
heuristics might provoke premature GCs )

Issue: -
The issue is due to late update of  average threads count
The method "global_stats()->allocating_threads_avg()"  always returns 1
until first young GC happens.

ThreadLocalAllocBuffer::initial_desired_size() returns  "init_sz=
tlab_capacity/ allocating_threads_avg *target_refills"

  i.e  init_sz = tlab_capacity/1*50.

Due to above calculation young GC happens before creating first 50 threads.

Issue happens with below command in jdk11 :-
$java -Xmn3520m -Xms3584m -Xmx3584m -XX:+PrintGC -XX:+UseParallelOldGC
-XX:+UseParallelGC Threads 64
[0.001s][warning][gc] -XX:+PrintGC is deprecated. Will use -Xlog:gc instead.
[0.004s][info   ][gc] Using Parallel
[0.209s][info   ][gc] GC(0) Pause Young (Allocation Failure)
2640M->1M(3144M) 14.863ms

Proposed Solution:
The variable "GlobalTLABStats:: _allocating_threads"  should be updated
with each thread creation.
So incremented  "GlobalTLABStats:: _allocating_threads" inside
ThreadLocalAllocBuffer::initialize ( call stack:- Thread ->
initialize_tlab() ->  tlab().initialize() ) .

Please find the patch below.

======================== PATCH ==========================================

diff -r d12828b7cd64 src/hotspot/share/gc/shared/threadLocalAllocBuffer.cpp
--- a/src/hotspot/share/gc/shared/threadLocalAllocBuffer.cpp    Wed Jun 13
10:15:35 2018 +0200
+++ b/src/hotspot/share/gc/shared/threadLocalAllocBuffer.cpp    Wed Jun 13
05:08:01 2018 -0500
@@ -192,7 +192,8 @@
   initialize(NULL,                    // start
              NULL,                    // top
              NULL);                   // end
+  global_stats()->update_allocating_threads();
+  global_stats()->publish();

   // Following check is needed because at startup the main


Roshan Mangal
MTS Software Engineer at AMD

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