Question about madvise(DONTNEED) in glibc malloc

[KOSAKI Motohiro]

>>>       MADV_DONTNEED
>>>              Do  not  expect  access  in the near future.  (For the time
>>>              being, the application is finished with the given range, so
>>>              the  kernel can free resources associated with it.)  Subse-
>>>              quent accesses of pages in this  range  will  succeed,  but
>>>              will  result  either  in  re-loading of the memory contents
>>>              from the underlying mapped file (see mmap(2)) or zero-fill-
>>>              on-demand pages for mappings without an underlying file.
>>
>> Current implemantation cleary does as document. It's not a bug and we don't plan to
>> change never.
> 
> I didn't imply that it's any kind of bug in the kernel either.
> 
> Why can't we extend the TRIM_THRESHOLD to non-main arenas?  We
> sacrifice the extra address space we could have given back to the
> kernel by not getting rid of a heap when we could, but that's a much
> better compromise IMO.

Yeah, I haven't consider it. but it can, I think.


> I agree with you when you say that the pages we just gave back with
> MADV_DONTNEED are the ones most likely to be needed when we decide to
> extend the heap next and in that context, it is a suboptimal use of
> MADV_DONTNEED.  However, the idea here is to release resources
> periodically to avoid monopolizing it.  Maybe we could always only
> trim half of TRIM_THRESHOLD and keep the other half as a cache.  Given
> the semantics of dynamic thresholds, we would effectively be keeping a
> cache for the next expected allocation size - the TRIM_THRESHOLD is
> dynamically set to twice of MMAP_THRESHOLD on freeing of a mapped
> location.

That's definitely better than current. but I'm incline to solve the issue
completely by introducing new kernel feature.