throwing static exceptions sometimes VERY slow!

Wed Mar 1 12:47:52 UTC 2017

Hi,

here are the general system infos:

uname -a:

Linux ber-28753 3.16.0-4-amd64 #1 SMP Debian 3.16.7-ckt20-1+deb8u3 (2016-01-17) x86_64 GNU/Linux

lscpu:

Architecture:          x86_64
CPU op-mode(s):        32-bit, 64-bit
Byte Order:            Little Endian
CPU(s):                4
On-line CPU(s) list:   0-3
Thread(s) per core:    1
Core(s) per socket:    4
Socket(s):             1
NUMA node(s):          1
Vendor ID:             GenuineIntel
CPU family:            6
Model:                 58
Model name:            Intel(R) Core(TM) i5-3450 CPU @ 3.10GHz
Stepping:              9
CPU MHz:               2094.558
CPU max MHz:           3500.0000
CPU min MHz:           1600.0000
BogoMIPS:              6186.30
Virtualization:        VT-x
L1d cache:             32K
L1i cache:             32K
L2 cache:              256K
L3 cache:              6144K
NUMA node0 CPU(s):     0-3

java -version:

java version "1.8.0_102"
Java(TM) SE Runtime Environment (build 1.8.0_102-b14)
Java HotSpot(TM) 64-Bit Server VM (build 25.102-b14, mixed mode)

Rest of the answers inline.

BR

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Dr. Jacob Wieland

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________________________________
From: Vitaly Davidovich <vitalyd at gmail.com>
Sent: Tuesday, February 28, 2017 1:46:09 PM
To: David Holmes
Cc: Wieland, Jacob; hotspot-dev at openjdk.java.net
Subject: Re: throwing static exceptions sometimes VERY slow!

On Tue, Feb 28, 2017 at 7:37 AM, David Holmes <david.holmes at oracle.com<mailto:david.holmes at oracle.com>> wrote:
Hi,

On 28/02/2017 9:02 PM, Wieland, Jacob wrote:
Hi,

I am observing a very strange behavior.

In our generated code (due to various reasons I won't go into here unless I have to, but trust me, they are legit), we throw static exceptions for control flow purposes, This seems to work fine and without performance loss most of the time. However, we are observing now that every few seconds, a throw sometimes takes up between 1,5 and 2.5 seconds! (in contrast to the normal almost non-measurable time).

Update: it seems that the observed behavior was kind of related with some extraneous circumstances (the computer seems to have been more lagged than usual at that point). By now, I can only observe throws that take up to 20 milliseconds (which opposed to the normal time still seems slow).

It does not seem to be GC related, the only idea that I have is the jitter.

What jitter?
I'm assuming Jacob is referring to the JIT.  But yes, he needs to provide more information.

In particular, it would be good to know the following, as David mentioned, (off the top of my head):
1) JVM cmdline flags

-Xdebug -Xrunjdwp:transport=dt_socket,address=8001,server=y,suspend=y

2) Type of exception thrown (with a stacktrace or not)

Exception class derived from java.lang.Error that overrides fillInStackTrace() with 'return this', so stackless. Same instances stored in static final fields are always thrown.

3) Is the call stack depth about the same during the slow and fast throws?

It is always exactly the same stack, as it happens in a loop.

4) Is the exception thrown frequently or infrequently?

In the test where this was observed, the exception was thrown every 40ms, so, I guess, frequently.

5) Is there -XX:+PrintCompilation output available around the time when the slowdown is observed

Unfortunately not, and at the moment, I cannot reproduce the behavior.

I made a mockup-test (see attachment) that mimicks kind of what my generated code does and adds measurements before throw and after catch.
Mostly, in my experiments with this, the throw takes on average 20 ns, but between a 1 and 3 per million iterations take longer than 1 ms, on average around 7 ms. It can be observed that doing a garbage collection in a different java process (namely my eclipse that started the test) greatly influences the frequency of the > 1 ms throws, so maybe it is simply a load issue. Using multiple threads in parallel also seems to heighten the

Also, when introducing an additional sleep between iterations (like in my original scenario where I was waiting for a timer to timeout before executing the construct that used the throw), I cannot reproduce such outliers at all.

So, my question is. Is this a known (and for some strange reason maybe even accepted) behavior or is this a bug that I should file with Oracle (or you guys).

You really are not giving us anything to go on here. How are you observing this slowness? Exactly how do you throw? What exactly are you measuring? What's the execution context, the machine, processors etc etc etc.

I was observing the slowness in a testcase where I wanted regular intervals of 40 ms between sending a message via UDP but frequently (every few seconds) got a lag of around 1.5 seconds (and a few days later 0.5 seconds) before the timer which timed out at the correct time was noted.

When eliminating one kind of such thrown control exception, the next kind of thrown control exception (from a surrounding construct) was having the same effect which actually led me to the slow-throw-hypothesis. Eliminating all exceptions in the small example confirmed this hypothesis, but going that route would take a lot of effort for all cases and introduce other additional imperformances (as I basically have to mock the exception throwing via cascading return and if-checks on every level) which I would like to avoid.

Regards,
David

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