[OpenJDK 2D-Dev] RFR: 8240654 : ZGC can cause severe UI application repaint issues

Philip Race philip.race at oracle.com
Wed Jun 10 20:48:27 UTC 2020

Bug: https://bugs.openjdk.java.net/browse/JDK-8240654
Webrev: http://cr.openjdk.java.net/~prr/8240654/index.html

This is for JDK 15 so review ASAP please since RDP 1 and the test cycle 
are looming.

This is not a fix for a JDK bug. It is a bunch of workarounds for a 
Microsoft Windows bug affecting
GDI in the context of ZGC (http://openjdk.java.net/jeps/333).
Some extra details about the Windows bug at the end, but first the 
technical details of the fix.

With ZGC's memory allocation requirement of reserving memory in 2Mb 
chunks  some Windows GDI
functions, mostly involving some bitmaps APIs may return a failure code 
(ie fail!)
This typically occurs when Java heap memory is used for a Java image and 
then in a JNI
call we use GetPrimitiveArrayCritical so that Java heap allocated memory 
is passed to a GDI
function AND the Java heap memory spans one of the 2Mb boundaries.
This is very easy to trigger in almost any Java UI app if the window is 
of a large enough (ie typical) size.
NB: if you have an Nvidia or ATI card, then you won't see it, because 
the D3D pipeline doesn't
call the affected method but if you have an Intel chip as do 90% (?) of 
laptops you will see it.
There are also several other places we found that are affected. Printing 
is the other one
somewhat easy to trigger. The others : custom cursors and tray icons are 
less common.
The painful thing here is that there is no definitive list (a list of 
the known ones is below) of
affected Windows GDI APIs and we are just hunting around our code trying 
to see where it
might be side-swiped by this bug.

The basic approach in these workarounds is that for cases where 
performance does not matter we now copy
and for cases where performance does matter or larger amounts of memory 
is involved we check if
the return value of the GDI function indicates failure and then re-try 
with a copy of the heap memory.
Unless GDI was randomly failing already (unlikely) this should be a 
no-risk solution in the high profile cases.
We have done performance measurements on the important screen case and 
the failures
happen fast so the penalty is then in the re-try which is only if ZGC is 
Always copying the memory is slower (and memcpy is the slow operation) 
than an alternative approach
that "knows" about the memory allocation of ZGC but this coupling and 
the complexity seem like they aren't
worth it since I haven't seen any visible performance consequence. That 
can be revisited
some day if need be, but for now we have correctness which is the key as 
well as sufficient performance.

I've created an automated test for the most important on-screen case.
Also a manual printing test case which invokes ZGC is provided since 
there we also only
conditionally copy. In the other cases we now always copy so existing 
test cases should over those.

There is some clean up in this fix - one completely unused (provably so 
because it was #if'd out)
JNI method in awt_PrintJob.cpp is removed since it had code that looked 
like it needed a workaround,
which would be somewhat of a waste of effort.

the doPrintBand code and its callee bitsToDevice has code I think we can 
remove too since
I don't see how it ever gets executed (the top down case for 
browserPrint == true) but
I think I'll save that for a P4 follow-on since it does nothing that 
would be affected by this
Windows bug.

One oddity is the in the printing case I observed that some times the 
rendering is performed
even if an error code is returned. I don't know why, but in code we 
can't tell that it was actually
rendered and in any case there is no harm in repeating the call with 
copied memory.

We are right before the JDK15 stabilisation fork and this fix needs to 
go there and will
but the webrev is against jdk/client simply because jdk15 does not exist 
yet !

Please test and review ASAP.

About the bug:
Microsoft has acknowleged the bug and will publish a knowledge base 
article about it
but a fix may show up only in a future version of Windows. Not, it 
seems, any time soon.
Below is a list of potentially affected GDI APIs. Per microsoft whether 
it actually manifests in
any specific case depends on "branching"














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