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From Salikh Zakirov <Salikh.Zaki...@Intel.com>
Subject [design] Class unloading: false sharing of vtable marks
Date Tue, 14 Nov 2006 10:25:29 GMT
As we discussed before, the VTable marks approach [1] has a "false sharing" problem
on a multiprocessor:

when one thread is writing to vtable mark, it is invalidating respective cache line
in other processor caches. Meanwhile, since gcmaps, located near vtable marks,
are loaded frequently during heap tracing, the same cache line will be loaded
and invalidated repeatedly, leading to huge load to memory bus and harming performance.

*Illustration*: original "VTable marks" suggestion applied to current DRLVM object layout.

   object            VTable                   gcmap
 +--------+        +-----------+            +------------------+
 | VT ptr |------->| gcmap ptr |----------->| offset of ref #1 |
 |  ...   |        |   mark    |            | offset of ref #2 |
 +--------+        +    ...    |            |       ...        |
                   +-----------+            |        0         |
                                            +------------------+

I would like suggest solution to false sharing problem using additional
level of indirection, that is, to store the _pointer to mark word_  in VTable
rather than mark word itself.

*Illustration*: "indirect VTable marks" suggestion

   object            VTable                   gcmap
 +--------+        +-----------+            +------------------+
 | VT ptr |------->| gcmap ptr |----------->| offset of ref #1 |
 |  ...   |        |  mark ptr |---,        | offset of ref #2 |
 +--------+        +    ...    |   |        |       ...        |
                   +-----------+   |        |        0         |
                                   |        +------------------+
                                   v
                              [mark word]

I do not think this will hurt performance significantly in comparison with original
"vtable marks" approach, because, additional load of mark_ptr is very likely
to be served from the first-level cache, because it happens at the same time
as gcmap_ptr load. (If the mark_ptr is loaded first, then subsequent load of gcmap_ptr
will be served from cache, so no additional memory load overhead anyway).

In current DRLVM design [2], each VTable already have pointers to native Class structure:

    Class* clss;

It looks like the same pointer can be reused for VTable mark word, if we allocate
VTable mark word as the first word of struct Class.
In this way, even the size of VTable structure will not be changed comparing
to current size. The resulting object layout diagram would be

*Illustration*: "indirect VTable marks stored in struct Class"

   object            VTable                   gcmap
 +--------+        +-----------+            +------------------+
 | VT ptr |------->| gcmap ptr |----------->| offset of ref #1 |
 |  ...   |        |  clss ptr |---,        | offset of ref #2 |
 +--------+        +    ...    |   |        |       ...        |
                   +-----------+   |        |        0         |
                                   |        +------------------+
                                   v
                             +-----------+
                             | mark word |
                             |    ...    |
                             +-----------+ 
                              struct Class


Robin suggested "side byte-map" as another solution to the same false sharing problem. 
As I do not completely understand how this side byte-map would be implemented, 
I do not know if it is similar to this suggestion.

Robin, could you comment on it?

[1] http://wiki.apache.org/harmony/ClassUnloading
[2] http://svn.apache.org/viewvc/incubator/harmony/enhanced/drlvm/trunk/vm/vmcore/include/vtable.h?view=co

(*
This is a follow-up to design proposals at 
http://wiki.apache.org/harmony/ClassUnloading

I am starting new discussion because mailing list is a better means for discussion
than Wiki. After we come to conclusion, I will log it to the wiki page.
*)



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