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Re: [RFC] Calling realloc vs. mallo/memcpy/free and _IO_vasprintf.


On 11/26/2015 10:14 PM, Siddhesh Poyarekar wrote:
> On Thu, Nov 26, 2015 at 09:34:29PM -0500, Carlos O'Donell wrote:
>> While answering a question on libc-help about asprintf I happened
>> to look at _IO_vasprintf and noticed some code which tries to
>> decide if it should do a realloc vs. malloc/memcpy/free.
>>
>> In this particular case we should always be shrinking the buffer.
>> For example _IO_vfprintf may have expanded the stream buffer but
>> now no longer needs the extra space. If we are growing the buffer
>> then something is very very wrong since that means _IO_vpfrintf
>> will have failed due to lack of memory, and we handled that case
>> earlier in _IO_vasprintf.
>>
>> So again, if we are only shrinking the buffer, then realloc has
>> to be the most optimal operation in all cases. It would seem to
>> me that an allocation shrink like this would almost never result
>> in an allocation move, but rather malloc would just split the
>> allocation and reuse the remainder for other malloc calls.
>>
>> So why the micro-optimization?
>>
>> Is my assumption of always shrinking the buffer wrong?
>>
>> If my assumption is right, and my logic is right, wouldn't the
>> following patch be a natural cleanup?
> 
> Why would malloc+memcpy+free be faster than a realloc, which does the
> same thing internally, but without adding the extra PLT and avoiding
> memcpy for smaller copies?  I'd say just drop that bit and stick to
> realloc.

Agreed. It appears to me that _int_realloc always splits the allocated
region, and frees the remainder if >= MINSIZE (non-mmaped case), which
is a faster operation. In the mmaped case we call mremap which will
shrink the mapping, which is a syscall, and in that case *might* be
slower tahn malloc/memcpy/free due to the cost of entering the kernel
and the work needing to be done. However, I would argue that
malloc/memcpy/free in the mean performance case is likely to be a
lot more work than just realloc.

Cheers,
Carlos.



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