Driver question

Sean Hinde <>
Tue Aug 28 20:41:11 CEST 2001


Hi,

A further question about the suggestion below:

> Yes they do. They might be exactly what you need. The driver binaries
> are reference counted, and when you call driver_allocate_binary(),
> refcnt is set to 1 (only the driver is holding the binary).
> 
> When you later call driver_output_binary(), refcnt is 
> incremented to 2,
> and note that the driver must not change the contents from 
> now on, since
> erlang processes now can see the data and depend on this. The driver
> migth as well call driver_free_binary(), which just decrements refcnt.
> Only when refcnt decrements to 0, the binary is actually freed.
> 
> Every erlang process that now receives the binary will 
> increment refcnt,
> and when the binary is garbage collected and found to be unused refcnt
> is decremented and finaly freed by the last erlang processe 
> that garbage
> collects it.

I have played with this somewhat.. I allocate the binary in the driver and
send the result to erlang. The refc is now 2 (although it sometime goes to 3
for a short time?). I do not free it at this time as I want to read the refc
in a future call to my driver and then if it is back to 1 I can free up a
whole load of other stuff as well as the binary.

I have a fixed size array of handles so it is possible for them all to be
used up in which case I return "congestion".

The behaviour I can't quite figure out though is that of
erlang:garbage_collect(). If I call this, the binaries are not immediately
reduced to refc = 1. It takes something like 10 seconds before any of them
are freed and then some appear to never be freed.

My binaries are 2 octets long.

Any thoughts on the behaviour of the garbage collector in this situation?

Thanks,
Sean



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