[Vm-dev] [COG] should cog unlinkSends in #flushMethodCacheFrom:to:

[Mariano Martinez Peck]

On Fri, Dec 31, 2010 at 7:25 PM, Eliot Miranda <eliot.miranda at gmail.com>wrote:

>
> Hi Mariano,
>
> On Fri, Dec 31, 2010 at 7:48 AM, Mariano Martinez Peck <
> marianopeck at gmail.com> wrote:
>
>>
>> Hi Eliot. I am very newbie in all this stuff, so I would directly ask you
>> if you could explain me my doubt ;)
>>
>>
>> StackInterpreter >>  flushMethodCacheFrom: memStart to: memEnd
>>     "Flush entries in the method cache only if the oop address is within
>> the given memory range.
>>     This reduces overagressive cache clearing. Note the AtCache is fully
>> flushed, 70% of the time
>>     cache entries live in newspace, new objects die young"
>>     | probe |
>>     probe := 0.
>>     1 to: MethodCacheEntries do: [:i |
>>             (methodCache at: probe + MethodCacheSelector) = 0
>>                 ifFalse: [((((self oop: (methodCache at: probe +
>> MethodCacheSelector) isGreaterThanOrEqualTo: memStart)
>>                                         and: [self oop: (methodCache at:
>> probe + MethodCacheSelector) isLessThan: memEnd])
>>                                     or: [(self oop: (methodCache at: probe
>> + MethodCacheClass) isGreaterThanOrEqualTo: memStart)
>>                                             and: [self oop: (methodCache
>> at: probe + MethodCacheClass) isLessThan: memEnd]])
>>                                 or: [(self oop: (methodCache at: probe +
>> MethodCacheMethod) isGreaterThanOrEqualTo: memStart)
>>                                         and: [self oop: (methodCache at:
>> probe + MethodCacheMethod) isLessThan: memEnd]])
>>                         ifTrue: [methodCache at: probe +
>> MethodCacheSelector put: 0]].
>>             probe := probe + MethodCacheEntrySize].
>>     1 to: AtCacheTotalSize do: [:i | atCache at: i put: 0]
>>
>>
>> But if I see
>>
>> CoInterpreter >> flushMethodCache
>>     "Flush the method cache. The method cache is flushed on every
>> programming change and garbage collect."
>>
>>     1 to: MethodCacheSize do: [ :i | methodCache at: i put: 0 ].
>>     lastMethodCacheProbeWrite := 0. "this for primitiveExternalMethod"
>>     cogit unlinkAllSends
>>
>>
>> So....my newbie question is just why the "cogit unlinkAllSends" is not
>> need in Cog?   I mean, why cog doesn't need to do:
>>
>> CoInterpreter >>  flushMethodCacheFrom: memStart to: memEnd
>>    super flushMethodCacheFrom: memStart to: memEnd .
>>    self unlinkAllSends.
>>
>
> Good question.  The answer is that  flushMethodCacheFrom:to: is being used
> to cheaply avoid having to remap entries in the method cache.  It gets
> called from mapPointersInObjectsFrom:to: during GC (which moves objects) and
> which could  actually remap the entries in the method cache instead of just
> throwing them away.  The issue for remapping is that there is no ordering of
> the objects in the method cache so one has to scan all of its entries, but
> since its small that's not likely to be an issue.  In any case, throwing
> away or unlinking sends in Cog is much more expensive, so I don't do it.
>
> Instead, Cog object references are updated in mapInterpreterOops (also
> called from mapPointersInObjectsFrom:to:) and high cost is avoided by
> maintaining a flag, inFullGC, that tells Cog whether it has to remap all
> object references or just young object references, and a list of Cog methods
> containing young references (see selectors matching youngReferrers).  So
> during a young space scavenge when doing mapMachineCode Cog only looks at
> the methods containing young referrers, and typically there aren't any.
>
>
Ahh okok, I think I got it :)
Thanks for the explanation Eliot!

Mariano




>> Thanks for the explanation in advance,
>>
>> Mariano
>>
>>
>>
>
>
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