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From Tim Ellison <t.p.elli...@gmail.com>
Subject Re: [drlvm] dynamic object layout
Date Tue, 07 Nov 2006 21:32:53 GMT
Geir Magnusson Jr. wrote:
> 
> Alexei Fedotov wrote:
>> Weldon,
>>
>> I agree with you that it is nearly impossible to achieve stability for
>> a branch under active development.
>>
>>  From the other side, adding new features is fun, and also has a reason
>> behind it. If we strive for a complete implementation of J2SE, we
>> cannot avoid this type of activity.
>>
>> So my suggestion is to create separate branches for new features which
>> could be merged into the main branch when mature enough to achieve an
>> appropriate level of stability. What do you think?
> 
> Well, there's a couple of things here.  Any committer is free to go off
> into a sandbox to do something radical.  However, there are features we
> simply need - class unloading, for example - that aren't new features
> being done just for fun.
> 
> Things are complicated and we've seen how some features from the past,
> say the TM or invocation API, were done off in a corner, that led to two
> problems when brought forward -
> 
> 1) There were lots of others that had useful input who weren't able to
> contribute until the feature was finished  and
> 
> 2) The iterations of discussion about the patch while ongoing progress
> was happening in the trunk made the big patches stale, which made it
> hard for people to examine, test and comment on.
> 
> I think that for something like this, we should evaluate the "new ideas"
> on the merit, and decide if it's critical to our goal of a competitive,
> compatible Harmony v1.0 (for example, class unloading) or simply a
> nice-to-have improvement (GCv5, maybe).
> 
> We have a really difficult job to do in the next 7.5 months - to get to
> a compatible 1.0* - so I'd like to encourage people to remain as focused
> as we can to get to that point.  That doesn't mean this isn't fun, but
> the way I see it, we have a few focused months of efforts before we
> begin TCK testing, and we probably need to make some hard choices to
> delay stuff.  We're a mighty community, but a relatively small one, so
> the more of us rowing in the same direction, the better.
> 
> So if JVMTI is slow?  What's the tradeoff?  My persoal perference would
> be to take stability for now, and revisit the JVMTI  performance later...

Just to add my 2c, in that I concur with this position.  There has to be
a judgement call on each of the new areas of functional improvement to
decide whether it will further disrupt improved stability goals.  In
general it is preferable to be solid but functionally incomplete rather
than vice versa.

Regards,
Tim

> geir
> 
> * Yeah, I dream of Harmony as the first compatible open source
> implementation of the JDK, beating Sun...
> 
> 
>>
>> Alexei
>>
>> On 11/3/06, Weldon Washburn <weldonwjw@gmail.com> wrote:
>>> Salikh,
>>>  I glanced at the patch.  What you propose below looks reasonable.  I
>>> really
>>> don't see any other way to do it and still get "usable" performance.
>>>
>>> All,
>>> My only worry is disturbing highly critical code like object layout. 
>>> Given
>>> that this JIRA has been open a long time, I guess its OK to apply the
>>> patch.  At some point, we need to stop adding functionality and focus on
>>> stability.
>>>
>>>
>>>
>>> On 11/3/06, Salikh Zakirov <Salikh.Zakirov@intel.com> wrote:
>>> >
>>> > Hi,
>>> >
>>> > I am currently continuing to work on improving JVMTI Heap Iteration
>>> > (HARMONY-1635),
>>> > particularly, tagging objects.
>>> >
>>> > The use case that I've heard of is tagging *all* objects for the
>>> purpose
>>> > of memory
>>> > profiling. According to what I've heard it causes 60x slowdown on
>>> Sun's
>>> > VM.
>>> > However, the initial tags implementation that I've uploaded to
>>> > HARMONY-1635
>>> > is far worse, as it uses linear search for get/set tag operations.
>>> >
>>> > (* for those who didn't read JVMTI spec, tags are jlong (8 byte
>>> integer)
>>> > values,
>>> > which can be attached to arbitrary objects in get/set manner *)
>>> >
>>> > The alternative approach I came up with is to use (mostly) constant
>>> time
>>> > algorithms
>>> > for get/set operations, is to store a tag pointer in each object.
>>> > Storing tag itself in an object is not an option, as JVMTI requires to
>>> > send
>>> > OBJECT_FREE events with tags for each reclaimed objects, and this
>>> > information would not be
>>> > available if the tag would be reclaimed together with the object.
>>> >
>>> > However, since the general consensus was that increasing object
>>> header is
>>> > highly undesired,
>>> > I've tried to implement the _conditional_ increase in object header.
>>> > Additional object header field is allocated in case JVMTI Agent has
>>> > requested
>>> > can_tag_objects capability.
>>> >
>>> > The modified object layout I used is as follows:
>>> >
>>> > +-------------------+
>>> > |   VTable pointer  |
>>> > +-------------------+
>>> > |      lockword     |
>>> > +-------------------+
>>> > |   [array length]  |
>>> > +-------------------+
>>> > |   [tag pointer]   |
>>> > +-------------------+
>>> > |    [padding]      |
>>> > +-------------------+
>>> > | fields or elements|
>>> > |       ...         |
>>> > +-------------------+
>>> >
>>> > Where [array length] is only present in array objects,
>>> > [tag pointer] is only present when can_tag_capability has been
>>> enabled at
>>> > startup
>>> > [padding] is only present in arrays of longs and doubles for natural
>>> > 8-byte alignment.
>>> >
>>> > VTable pointer is really uint32 offset on em64t/x86_64 and ipf/ia64.
>>> >
>>> > The only difference with current object layout is introduction of tag
>>> > pointer field.
>>> >
>>> > I've modified gc_cc to take the changed dynamic object layout into
>>> > account,
>>> > and surprisingly it took only one modification:
>>> >
>>> > * use VM function vector_first_element_offset_unboxed() instead of
>>> > hardcoding
>>> > first array element offset. This is done once for each class done at
>>> > loading stage,
>>> > and gc_cc caches this offset for later uses.
>>> >
>>> > I've experimented with putting tag pointer at fixed location before
>>> array
>>> > length,
>>> > but it looks expensive, as it will add one more read to GC array
>>> scanning,
>>> > and
>>> > we obviously do not want optimize at the expense of common case.
>>> >
>>> > The latest version of the patch is attached to HARMONY-1635 (
>>> > heap-iteration-optimized.patch),
>>> > I would appreciate any comments and concerns.
>>> >
>>> >
>>> >
>>>
>>>
>>> -- 
>>> Weldon Washburn
>>> Intel Enterprise Solutions Software Division
>>>
>>>
>>
>>
> 

-- 

Tim Ellison (t.p.ellison@gmail.com)
IBM Java technology centre, UK.

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