Re: PING [patch] malloc: add mxfast tunable

[Carlos O'Donell <carlos at redhat dot com>]

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?

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)


--
Cheers,
Carlos.