Re: PING [patch] malloc: add mxfast tunable

[Florian Weimer <fweimer at redhat dot com>]

* Carlos O'Donell:

> On 8/8/19 5:18 PM, Florian Weimer wrote:
>> * Carlos O'Donell:
>>
>>> On 8/8/19 5:04 PM, Florian Weimer wrote:
>>>> * DJ Delorie:
>>>>
>>>>>> +@deftp Tunable glibc.malloc.mxfast
>>>>>> +One of the optimizations malloc uses is to maintain a series of ''fast
>>>>>> +bins'' that hold chunks up to a specific size.  The default and
>>>>>> +maximum size which may be held this way is 80 bytes on 32-bit systems
>>>>>> +or 160 bytes on 64-bit systems.  Applications which value size over
>>>>>> +speed may choose to reduce the number of fast bins with this tunable.
>>>>>> +Note that the value specified includes malloc's internal overhead,
>>>>>> +which is normally the size of one pointer, so add 4 on 32-bit systems
>>>>>> +or 8 on 64-bit systems to the size passed to @code{malloc} for the
>>>>>> +largest bin size to enable.
>>>>>> +@end deftp
>>>>
>>>> I think the quotes are wrong, they should be `` '' (ASCII, Texinfo will
>>>> transform them).  Or you could use @dfn and add an index entry.
>>>>
>>>> Does the fastbin range depend on pointer or size or minimum malloc
>>>> alignment?  The latter is 16 bytes on some 32-bit architectures, even
>>>> though it is usually 8 bytes.
>>>
>>> Entirely on SIZE_SZ.
>>>
>>> /* The maximum fastbin request size we support */
>>> #define MAX_FAST_SIZE     (80 * SIZE_SZ / 4)
>>
>> Huh.  Doesn't this mean we have unused fastbins because there is never
>> an allocation of the appropriate size to fill it?
>
> The fastbin index code already offsets the first bins because they
> are unindexable? Is that what you mean?

With 16 byte alignment, we have possible chunk sizes 16, 32, 48, 64, 80.

> 1571 /* offset 2 to use otherwise unindexable first 2 bins */
> 1572 #define fastbin_index(sz) \
> 1573   ((((unsigned int) (sz)) >> (SIZE_SZ == 8 ? 4 : 3)) - 2)

These sizes result in fastbin indices 0, 2, 4, 6, 8.  The odd indices
are unused, I think.

Thanks,
Florian