[Vm-dev] Igor's fast become for CompiledMethods in Cog

Tue Jan 31 23:28:20 UTC 2012

On Tue, Jan 31, 2012 at 3:13 PM, Mariano Martinez Peck <
marianopeck at gmail.com> wrote:

>
>
>
> On Tue, Jan 31, 2012 at 11:42 PM, Eliot Miranda <eliot.miranda at gmail.com>wrote:
>
>>
>>
>>
>> On Tue, Jan 31, 2012 at 12:41 PM, Mariano Martinez Peck <
>> marianopeck at gmail.com> wrote:
>>
>>>
>>>
>>>
>>> On Tue, Jan 31, 2012 at 9:28 PM, Eliot Miranda <eliot.miranda at gmail.com>wrote:
>>>
>>>>
>>>>
>>>>
>>>> On Tue, Jan 31, 2012 at 12:19 PM, Igor Stasenko <siguctua at gmail.com>wrote:
>>>>
>>>>>
>>>>> On 31 January 2012 20:50, Eliot Miranda <eliot.miranda at gmail.com>
>>>>> wrote:
>>>>> >
>>>>> >
>>>>> >
>>>>> > On Tue, Jan 31, 2012 at 11:22 AM, Mariano Martinez Peck <
>>>>> marianopeck at gmail.com> wrote:
>>>>> >>
>>>>> >>
>>>>> >> Hi Eliot. Me again :)   I was checking the changes Igor did some
>>>>> time ago for the fast become where he basically swapped the bytes contents
>>>>> between the objects when they were the same size and same header type. He
>>>>> put such code in separate primtives and some changes in the image side to
>>>>> call them. I have just played with them and they seem to work. I have 2
>>>>> questions for you:
>>>>> >>
>>>>> >> 1) Do you think that this new fast become can have problems when
>>>>> becoming CompiledMethods? I am asking because of the JIT/Pic. Maybe I need
>>>>> a flushCache or something?
>>>>> >
>>>>> >
>>>>> > Yes, almost certainly.  You'd want to do a flushCache on both
>>>>> methods.
>>>>> >
>>>>> are there other object types which we need to be careful with?
>>>>>
>>>>
>>>> There are a few.  e.g. the Array literals in named primitives (because
>>>> they hold target function pointers).  CompiledMethods (because they may
>>>> have associated machine code).  Contexts (because they may have associated
>>>> stack frames).
>>>>
>>>>
>>>
>>> Eliot, I don't understand why we have these problems with the "fast
>>> become" but not with the normal one. What happens wich each of your
>>> examples with the normal become? how are they solved?
>>>
>>
>> The "slow" become is implemented in terms of the GC's pointer-forwarding
>> mechanism, which is used in normal garbage collection, not just become.
>>  This machinery is the ObjectMemory>remap: machinery.  The JIT implements
>> the same mapping machinery for literal objects embedded in machine code.
>>  These include not just literals but also classes in inline-caches.  So it
>> would seem that implementing markObject: and remap: for literals in jitted
>> methods is all one needs to support GC and become:.  In fact, life is more
>> complex because there is an optimization in the JIT to avoid scanning all
>> of machine code on incremental GC.  The jit maintains a list of those
>> methods that contain references to young objects and only scans this list
>> on an incremental GC, and this list must be maintained correctly.  Hence
>> there are three different remap routines in the jit,
>>
>> Cogit>mapObjectReferencesInMachineCodeForIncrementalGC
>> "Update all references to objects in machine code for an incremental gc.
>>  Avoid scanning all code by using the youngReferrers list.  In an
>> incremental
>>  GC a method referring to young may no longer refer to young, but a method
>>  not referring to young cannot and will not refer to young afterwards."
>>
>> Cogit>mapObjectReferencesInMachineCodeForFullGC
>>  "Update all references to objects in machine code for a full gc.  Since
>>  the current (New)ObjectMemory GC makes everything old in a full GC
>>  a method not referring to young will not refer to young afterwards"
>>
>>
>> Cogit>mapObjectReferencesInMachineCodeForBecome
>>  "Update all references to objects in machine code for a become.
>>  Unlike incrementalGC or fullGC a method that does not refer to young
>>  may refer to young as a result of the become operation."
>>
>
>
>
> Aha. Ok. Now I see. So, let me see if I understand. So the problem of
> CompiledMethod gets fixed if we flush its cache. Right?
>

I hope so.  The current implementation reads

CoInterpreterPrimitives>primitiveFlushCacheByMethod
"The receiver is a compiledMethod.  Clear all entries in the method lookup
cache that
 refer to this method, presumably because it has been redefined, overridden
or removed.
 Override to flush appropriate machine code caches also."
super primitiveFlushCacheByMethod.
cogit unlinkSendsTo: self stackTop

That may not be enough.  The VM may have to throw the method away.  Tests
will show whether merely unlinking is sufficient.  The issue is that if the
method is being used then throwing it away may involve flushing the stack
activations of the method, and that makes the implementation much more
complex.

> Now...if we always send #mapObjectReferencesInMachineCodeForBecome   after
> the "fast become" we will be updating all literals from machine code
> methods.
>

Um, will it? The mapping is done only to references to objects that are
forwarded.  If that's going to do the trick then great.  But I don't know
enough about your fast become to know.

I didn't understand this one "  e.g. the Array literals in named primitives
> (because they hold target function pointers)"
>

Look at a method containing a named primitive that's in use and look at its
first literal.  e.g.

(StandardFileStream >> #primRead:into:startingAt:count:) literalAt: 1
#(#FilePlugin #primitiveFileRead 0 12)

That 12 is meaningful to the VM.  See primitiveExternalCall:

External primitive methods first literals are an array of
* The module name (String | Symbol)
* The function name (String | Symbol)
* The session ID (SmallInteger) [OBSOLETE]
* The function index (Integer) in the externalPrimitiveTable

> What happens with "Contexts (because they may have associated stack
> frames)."  ?  should we need to flush somehow or update stack frames?
>

Yes.  If you smash the state of a context that has an associated stack
frame then the VM will likely crash.  See senders of
externalDivorceFrame:andContext: to see where the VM disassociates contexts
and their stack frames when an access to a context (e.g. changing its stack
pointer or pc) necessitates it.

>
> Thanks!
>
>
>>
>>
>>>  Sorry for the noob question.
>>>
>>
>> It's a good question :)
>>
>>
>>>
>>>
>>>
>>>> because i was thinking to just put a check in fast-become prim and
>>>>> simply fail the prim if object type(s) to be swapped are not
>>>>> supported, so user will be forced to use slow good-old #become:
>>>>>
>>>>
>>>> I agree.  But you can do even better, by checking that the compiled
>>>> method has a machine-code version, and/or checking that a context is
>>>> "single" (has no associated stack state).  It doesn't need to fail if there
>>>> isn't any special state.  Identifying the named primitive linking literals
>>>> is more difficult...
>>>>
>>>>
>>> Ideally, I would love to be able to do the fast become for all of them,
>>> even if that implies doing something extra for special cass (like flushing
>>> method cache).
>>>
>>
>> As they say, don't get caught.
>>
>>
>>>
>>>
>>>
>>>>
>>>>> --
>>>>> Best regards,
>>>>> Igor Stasenko.
>>>>>
>>>>
>>>>
>>>>
>>>> --
>>>> best,
>>>> Eliot
>>>>
>>>>
>>>>
>>>
>>>
>>> --
>>> Mariano
>>> http://marianopeck.wordpress.com
>>>
>>>
>>>
>>
>>
>> --
>> best,
>> Eliot
>>
>>
>>
>
>
> --
> Mariano
> http://marianopeck.wordpress.com
>
>
>

-- 
best,
Eliot
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