[darcs-users] darcs patch: switch Darcs.Patch.FileName to be ByteString.Char8 int...

Simon Marlow marlowsd at gmail.com
Mon Sep 28 08:06:08 UTC 2009


On 25/09/2009 16:52, Duncan Coutts wrote:
> On Fri, 2009-09-25 at 14:58 +0100, Simon Marlow wrote:
>
>> The underlying problem in your example may be fragmentation, due to the
>> way that ByteStrings are pinned and hence hold on to the whole 4KB block
>> in which they were allocated until they die.  Duncan has been thinking
>> about how to improve the situation, but I'm not sure of the current
>> status - Duncan?
>
>
> It's a medium term project. There is no short term fix. The problem is
> using pinned heap arrays. Switching to unpinned would entail an almost
> complete rewrite of the bytestring implementation. That is something I
> would like to do however it's not quick, and it would have to pass the
> "dons" test, that the results are absolutely definitely as good as the
> current version. Don is understandably nervous about upsetting the
> existing mature implementation. That would also include the ability to
> mmap, which would be lost under a straightforward re-implementation to
> use unpinned arrays. Handling mmap'ed ByteArray# is another
> mini-project.

As a workaround, would it be possible to provide an unsafePack that 
gives you an unpinned ByteString, with the onus on the caller to ensure 
they never pass it to C?  Perhaps it would even be possible to check 
that it was never passed to C?  And it's only "safe" foreign imports 
that are dangerous, it's fine to pass an unpinned ByteArray to an unsafe 
foreign import (though that's probably not something we should rely too 
heavily on, in case we want to change it in the future).

Cheers,
	Simon


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