Re: Serious concurrency problems on fast systems

Robert Klemme <>
Tue, 08 Jun 2010 23:27:27 +0200
On 08.06.2010 05:39, Kevin McMurtrie wrote:

In article<>,
  Robert Klemme<> wrote:

On 07.06.2010 08:25, Kevin McMurtrie wrote:

In article<4c0c57a7$0$282$>,
   Arne Vajh=F8j<> wrote:

On 02-06-2010 01:45, Kevin McMurtrie wrote:

In article<4c048acd$0$22090$>,
    Kevin McMurtrie<> wrote:

I've been assisting in load testing some new high performance serv=


running Tomcat 6 and Java 1.6.0_20. It appears that the JVM or Li=

nux is

suspending threads for time-slicing in very unfortunate locations.=


example, a thread might suspend in Hashtable.get(Object) after a c=

all to

getProperty(String) on the system properties. It's a synchronized=

global so a few hundred threads might pile up until the lock holde=


resumes. Odds are that those hundreds of threads won't finish bef=


another one stops to time slice again. The performance hit has a =

ton of

hysteresis so the server doesn't recover until it has a lower load=


before the backlog started.

The brute force fix is of course to eliminate calls to shared
synchronized objects. All of the easy stuff has been done. Some
operations aren't well suited to simple CAS. Bottlenecks that are=


of well established Java APIs are time consuming to fix/avoid.

Is there JVM or Linux tuning that will change the behavior of thre=


time slicing or preemption? I checked the JDK 6 options page but =


find anything that appears to be applicable.

To clarify a bit, this isn't hammering a shared resource. I'm talk=


about 100 to 800 synchronizations on a shared object per second for=


duration of 10 to 1000 nanoseconds. Yes, nanoseconds. That should=


cause a complete collapse of concurrency.

But either it does or your entire problem analysis is wrong.

My older 4 core Mac Xenon can have 64 threads call getProperty(Stri=


on a shared Property instance 2 million times each in only 21 real
seconds. That's one call every 164 ns. It's not as good as
ConcurrentHashMap (one per 0.30 ns) but it's no collapse.

That is a call per clock cycle.

HotSpot has some (benchmark-driven?) optimizations for this case. It=


hard to not hit them when using simple tests on String and

What exactly do you mean by that? I can't seem to get rid of the
impression that you are doing the second step (micro optimization with=

JVM internals in mind) before the first (proper design and implementat=



Many of the basic Sun Java classes are synchronized.

Practically only old ones that you should not be using anymore

Properties is a biggie. A brute-force replacement of Properties caus=


the system throughput to collapse to almost nothing in Spring's
ResourceBundleMessageSource. There's definitely a JVM/OS problem. T=


next test is to disable hyperthreading.

As someone else (Lew?) pointed out it's a bad idea to always go to You should rather be evaluating them on startup an=


initialize some other data structure - if only to not always repeat
checking of input values over and over again.

The properties aren't immutable. The best feature of properties rather=

than hard-coded values is being able to update them in an emergency
without server restarts. Anyways, that was fixed by overriding every
method in Properties with a high-concurrency implementation. Too bad
Properties isn't an interface.

Well, then use an immutable Hash map as Lew suggested and store it via
AtomicReference. You get fast concurrent access and can update it at
any time.

Fixing every single shared synchronized method in every 3rd party
library could take a very, very long time.

I have no idea where you take that from. Nobody suggested fixing third
party libraries - if anything the suggestion was to use them properly.
Note that you can even use a Vector without too much performance
degradation if it is accessed by a single thread only (of course one
would use ArrayList then). It's not the libs - it's the way you use them=



remember.guy do |as, often| as.you_can - without end

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