[OpenJDK 2D-Dev] <AWT Dev> RFR: 8240654 : ZGC can cause severe UI application repaint issues
Sergey.Bylokhov at oracle.com
Thu Jun 11 01:50:04 UTC 2020
Just curious about what performance degradation we will have in the worst case.
On 6/10/20 3:57 pm, Kevin Rushforth wrote:
> Code changes look good.
> I verified that the bug happens on my Windows 10 laptop with SwingSet2 and with the LargeWindowPaintTest without the patch, and everything looks good with the patch.
> -- Kevin
> On 6/10/2020 1:48 PM, Philip Race wrote:
>> 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 enabled.
>> 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"
Best regards, Sergey.
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