Re: [PATCH] realloc: Return unchanged if request is within usable size

[Siddhesh Poyarekar <siddhesh@sourceware.org>]

On 2022-12-06 07:45, Wilco Dijkstra wrote:
> Hi Siddhesh,
> 
>>> We do need to handle the case where the size is smaller than the usable size since
>>> we'd still want to free the unused portion if it is a significant fraction of the total size.
>>
>> I'll make this a function of trim_threshold so that the reuse happens
>> only if the difference between the request and usable size is less than
>> the trim_threshold.
> 
> That's better, but that really only helps the mmap case, and all other reallocs will
> never release memory due to the 128KB default value of trim_threshold.

What I'm trying to ensure here is that holes are always less than the 
trim threshold.

> That's something we should explicitly decide/document and remove any redundant
> code from realloc as a result.

OK, I see the MINSIZE split is no longer relevant.

>>> Note the mmap code always tries to remap before checking the new size is smaller
>>> than usable size - so will end up remapping even if you downsize by one page.
>>
>> I don't see that; I've put the musable check before the
>> chunk_is_mmapped() check.  What am I missing?
> 
> I'm talking about the committed version, basically if there is any change in number of
> pages it will do a system call. With your change there must be at least 128KB worth of
> pages to be freed. We should do something similar for growing mmaps - large blocks
> should never grow/shrink a single page at a time.

Hmm, let me think about this.  I reckon this is going to need more 
sophisticated heuristics.

Thanks,
Sid