[squeak-dev] FFI pinning examples?

Eliot Miranda eliot.miranda at gmail.com
Fri Aug 25 14:21:22 UTC 2017

Hi Hannes,

> On Aug 25, 2017, at 1:47 AM, H. Hirzel <hannes.hirzel at gmail.com> wrote:
> What is memory pinning and when would I want to use it?

Pinning is the ability to tell the garbage collector not to move a specific object.  The GC moves objects when it scavenges surviving objects in new space (garbageCollectMost) and when it compacts old space as part of a "full GC" (garbageCollect).  If an object is pinned the GC arranges that the object is in old space, using a become if required, and sets the pinned flag bit in the object's header. The compactor then moves only unpinned objects, leaving pinned objects where they are.

Pinning is useful for passing objects to external code, for example to use as a buffer for copying data between Smalltalk and external code.  Since pinned objects don't move the external code can simply hold onto the address of the start of the object's body and write into the object, instead of having to track the object via a handle that points to a possibly moving object.

> --Hannes
>> On 8/25/17, Ben Coman <btc at openinworld.com> wrote:
>> On Fri, Aug 25, 2017 at 6:06 AM, Eliot Miranda <eliot.miranda at gmail.com>
>> wrote:
>>> Hi Phil,
>>>> On Thu, Aug 24, 2017 at 1:36 PM, Phil B <pbpublist at gmail.com> wrote:
>>>> I remember reading a while back that Spur added memory pinning and was
>>>> curious if any Squeak packages has made use of it yet? (Looking for an
>>>> example or two to get me started)
>>> The protocol i in Object, with the selectors pin, unpin and isPinned.
>>> You
>>> can send any of these to any non-immediate object.  It is your
>>> responsibility to avoid calling unpin until external code no longer holds
>>> references to any pinned object.  It is your responsibility to arrange
>>> that
>>> the objects are still accessible within the system so that they are not
>>> GCed while external code holds onto them.  Pinning is orthogonal to GC.
>>> It
>>> prevents the GC from moving objects, but does not prevent them from being
>>> GCed if no references to them exist in the Smalltalk system.  Note that
>>> also pinning may cause an object to move from newSpace to oldSpace, as
>>> objects are only pinball in oldSpace.  Pinning a new object causes it to
>>> become a copy of the object in oldSpace, so do not pass an object to
>>> external code and then pin it; it may move.
>>> So the idea is
>>> - create some objects you want to share with external code and pin them
>>> - pass them to that external code through the FFI
>>> - allow that external code to run (e.g spawn a thread in the external
>>> code, or make subsequent call though the FFI exercise the code)
>>> - shut down the external code
>>> - unpin your objects, or merely allow them to be GCed.
>>> Alternatively external code can pin the objects it is given, providing it
>>> updates their location afterwards.  See pinObject:.
>> Sorry I haven't looked at the implementation (and no Image near me right
>> now) but a random query crosses my mind,
>> does/can the function called externally return the new location, rather
>> than having to do this in two steps.
>> This might help ensure people are aware the new address should be dealt
>> with.
>> cheers -ben
>>> Note that you should use the interpreterProxy accessors
>>> firstIndexableField: and firstFixedField: to find out the start of the
>>> body
>>> of an object (pinned or otherwise).  byteSizeOf:, instanceSizeOf:,
>>> slotSizeOf: and stSizeOf: can all be used to find out various sizes.  See
>>> platforms/Cross/vm/sqVirtualMachine.h, which also provides pinObject: and
>>> unpinObject:.
>>> Clearly some work is needed to add documentation in the right places.
>>> LMK
>>> where you looked, where you would expect to find such information, etc.
>>> --
>>> _,,,^..^,,,_
>>> best, Eliot

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