possible CMS problem with C2

Tom Rodriguez tom.rodriguez at oracle.com
Thu Mar 31 17:57:41 PDT 2011


To close the loop on this, here's at least one case of this I found with a little awkery and inspection:

00c   B1: #     B151 B2 <- BLOCK HEAD IS JUNK   Freq: 1                                                                                           
00c     ! stack bang                                                                                                                                 
        SAVE   R_SP,-112,R_SP                                                                                                                         
018     SET    0xf92d4000,R_L1  !ptr                                                                                                                 
01c +   STW    R_I3,[R_I0 + #20]        ! ptr ! Field spec/benchmarks/_213_javac/FieldDefinition.documentation                                        
020     SRL    R_I0,#9,R_L0     ! Cast ptr R_I0 to int and shift                                                                                     
024 +   STB    #0,[R_L1 + R_L0] ! CMS card-mark byte 0                                                                                               
028     LDUW   [R_SP + #188],R_L0       ! spill                                                                                                       
02c +   STW    R_L0,[R_I0 + #28]        ! ptr ! Field spec/benchmarks/_213_javac/FieldDefinition.value                                               
030     LDUW   [R_SP + #184],R_L1       ! spill                                                                                                       
034 +   STW    R_L1,[R_I0 + #24]        ! ptr ! Field spec/benchmarks/_213_javac/FieldDefinition.exp                                                 
038     LDUW   [R_SP + #176],R_L7       ! spill                                                                                                       
03c +   STW    R_L7,[R_I0 + #36]        ! ptr ! Field spec/benchmarks/_213_javac/FieldDefinition.name                                                 
040 +   STW    R_I5,[R_I0 + #16]        ! ptr ! Field spec/benchmarks/_213_javac/FieldDefinition.type                                                 
044 +   STW    R_I4,[R_I0 + #12] ! Field spec/benchmarks/_213_javac/FieldDefinition.modifiers                                                         
048     STW    R_I2,[R_SP + #96]        ! spill                                                                                                       
04c +   STW    R_I2,[R_I0 + #32]        ! ptr ! Field spec/benchmarks/_213_javac/FieldDefinition.clazz                                               
050     STW    R_I1,[R_SP + #92]        ! spill                                                                                                       
054 +   STW    R_I1,[R_I0 + #8] ! Field spec/benchmarks/_213_javac/FieldDefinition.where                                                             
058     LDUW   [R_I5 + #8],R_L1 ! int ! Field spec/benchmarks/_213_javac/Type.typeCode                                                               
05c     NullCheck R_I5

If we don't eliminate any of the StoreCMs this code looks like this:

00c   B1: #     B152 B2 <- BLOCK HEAD IS JUNK   Freq: 1                                                                                           
00c     ! stack bang                                                                                                                                 
        SAVE   R_SP,-112,R_SP                                                                                                                         
018     SET    0xb53d2000,R_L2  !ptr                                                                                                                 
01c +   SRL    R_I0,#9,R_L1     ! Cast ptr R_I0 to int and shift                                                                                     
020     STW    R_I2,[R_I0 + #32]        ! ptr ! Field spec/benchmarks/_213_javac/FieldDefinition.clazz                                               
024 +   ADD    R_L2,R_L1,R_L0                                                                                                                         
028     STW    R_I5,[R_I0 + #16]        ! ptr ! Field spec/benchmarks/_213_javac/FieldDefinition.type                                                 
02c +   STB    #0,[R_L0]        ! CMS card-mark byte 0                                                                                               
030     LDUW   [R_SP + #176],R_L3       ! spill                                                                                                       
034 +   STW    R_L3,[R_I0 + #36]        ! ptr ! Field spec/benchmarks/_213_javac/FieldDefinition.name                                                 
038 +   STB    #0,[R_L0]        ! CMS card-mark byte 0                                                                                               
03c     LDUW   [R_SP + #184],R_L2       ! spill                                                                                                       
040 +   STW    R_L2,[R_I0 + #24]        ! ptr ! Field spec/benchmarks/_213_javac/FieldDefinition.exp                                                 
044 +   STB    #0,[R_L0]        ! CMS card-mark byte 0                                                                                               
048     LDUW   [R_SP + #188],R_L1       ! spill                                                                                                       
04c +   STW    R_L1,[R_I0 + #28]        ! ptr ! Field spec/benchmarks/_213_javac/FieldDefinition.value                                               
050 +   STB    #0,[R_L0]        ! CMS card-mark byte 0                                                                                               
054 +   STW    R_I3,[R_I0 + #20]        ! ptr ! Field spec/benchmarks/_213_javac/FieldDefinition.documentation                                        
058 +   STB    #0,[R_L0]        ! CMS card-mark byte 0                                                                                               
05c +   STB    #0,[R_L0]        ! CMS card-mark byte 0                                                                                               
060     STW    R_I1,[R_SP + #92]        ! spill                                                                                                       
064 +   STW    R_I1,[R_I0 + #8] ! Field spec/benchmarks/_213_javac/FieldDefinition.where                                                             
068     STW    R_I2,[R_SP + #96]        ! spill                                                                                                       
06c +   STW    R_I4,[R_I0 + #12] ! Field spec/benchmarks/_213_javac/FieldDefinition.modifiers                                                         
070     LDUW   [R_I5 + #8],R_L1 ! int ! Field spec/benchmarks/_213_javac/Type.typeCode                                                               
074     NullCheck R_I5

So it definitely occurs in the wild.  I think we're going to need to optimize away the redundant ones since they appear to be fairly common.  II see how to do it during final_graph_reshape so that we can accumulate the dependences and place the single one where it should go.

tom

On Mar 31, 2011, at 2:17 PM, Tom Rodriguez wrote:

> I filed 7032963 for this.
> 
> tom
> 
> On Mar 31, 2011, at 1:52 PM, Y. S. Ramakrishna wrote:
> 
>> Hi Tom --
>> 
>> On 03/31/11 12:41, Tom Rodriguez wrote:
>>> While looking at the code we generate for a simple program I noticed that we were eliminating card marks with CMS in a illegal fashion.  For this simple program:
>>> public class test {
>>>   static Object a1;
>>>   static Object a2;
>>>   static Object a3;
>>>   public static void main(String[] args) {
>>>       a1 = args;
>>>       a2 = args;
>>>       a3 = args;
>>>   }
>>> }
>>> we generate this:
>>> 000   B1: #     N1 <- BLOCK HEAD IS JUNK   Freq: 1
>>> 000     SAVE   R_SP,-72,R_SP
>>> 004     SET    precise klass test: 0x00843a50:Constant:exact *,R_L0     !ptr
>>> 00c +   SET    0xf92d4000,R_L2  !ptr
>>> 010 +   STW    R_I0,[R_L0 + #352]       ! ptr ! Field test.a3
>>> 014 +   STW    R_I0,[R_L0 + #348]       ! ptr ! Field test.a2
>>> 018 +   SRL    R_L0,#9,R_L1     ! Cast ptr R_L0 to int and shift
>>> 01c     STW    R_I0,[R_L0 + #344]       ! ptr ! Field test.a1
>>> 020 +   STB    #0,[R_L2 + R_L1] ! CMS card-mark byte 0
>>> 024     SETHI  #PollAddr,L0     ! Load Polling address
>>>       LDUW   [L0],G0  !Poll for Safepointing
>>>       RET
>>>       RESTORE
>>> 034 +   ! return
>>> 034
>>> which happens to be ok because the STB happens last but we don't actually have enough dependences to ensure that we get this schedule.  We emit 3 separate StoreCMs for each field references and each one has a dependence on the store that it covers.  What's going wrong is that we're allowing StoreCM to participate in the the store elimination in StoreNode::Ideal so we end up with this:
>>> 45     StoreP  ===  5  7  44  10  [[ 16  50 ]]  @precise klass test: 0x00843a50:Constant:exact+352 *, name=a3, idx=6;  Memory: @precise klass test: 0
>>> x00843a50:Constant:exact+352 *, name=a3, idx=6; !jvms: test::main @ bci:9
>>> 36     StoreP  ===  5  7  35  10  [[ 16 ]]  @precise klass test: 0x00843a50:Constant:exact+348 *, name=a2, idx=5;  Memory: @precise klass test: 0x008
>>> 43a50:Constant:exact+348 *, name=a2, idx=5; !jvms: test::main @ bci:5
>>> 25     StoreP  ===  5  7  24  10  [[ 16 ]]  @precise klass test: 0x00843a50:Constant:exact+344 *, name=a1, idx=4;  Memory: @precise klass test: 0x008
>>> 43a50:Constant:exact+344 *, name=a1, idx=4; !jvms: test::main @ bci:1
>>> 50     StoreCM ===  5  7  31  23  45  [[ 16 ]]  @rawptr:BotPTR, idx=Raw;  Memory: @rawptr:BotPTR, idx=Raw; !jvms: test::main @ bci:9
>>> The store to a3 has a StoreCM and it has killed the StoreCMs for a1 and a2 but it doesn't have a dependence on it.  Since the slices are independent the a3 operations could be scheduled before the store to a1 and a2.  It may be that this is rare in practice and/or the local schedule tends to put the StoreCM last but it's clearly wrong.  G1 is safe from this because the StoreCMs are never close enough to be eliminated.
>>> It's easy to fix but it might hurt CMS performance a bit.  It could also be done safely if the StoreCM could have dependences on multiple stores but since it's not using normal precedence edges I'm not sure how this would be implemented.
>>> Does this sound like any issues that have been seen in the past?
>> 
>> If it's been seen in the past, it has never been identified
>> as stemming from this kind of optimization / missing dependencies.
>> I have heard people (Vladimir, Igor, John Cuthbertson) express the lack of
>> confidence in the existence of sufficient dependencies in some of the
>> card-mark eliminations before.
>> 
>> There was a time way back when Mike Paleczny and Ross Knippel
>> had fixed issues stemming from illegal (for CMS) elision of card-marks
>> in the case of CMS, and we had also taken care to add some
>> missing ones in the runtime for perm objects, but this
>> one is new to me.
>> 
>> Yes, please fix this; even though i imagine this will be rare
>> because (1) such scheduling may be rare (2) it's only when this
>> intersects with precleaning that this would manifest, and if
>> CMS collections are infrequent, the error will be rare (3) the
>> lack of a card-mark for the elided/rescheduled stores may be
>> masked by a different store to an adjacent object, making an
>> actual crash rarer still. But still, thanks for finding the
>> issue and fixing it! It would be interesting to see how much
>> of a difference in performance the new previously missing
>> dependencies will make to real codes/workloads.
>> 
>> -- ramki
>> 
>>> tom
> 



More information about the hotspot-gc-dev mailing list