Re: Changing MMAP_THRESHOLD in malloc() implementation.

[ritesh sonawane <rdssonawane2317@gmail.com>]

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Thank you very much for instant reply..

Yes sir it is wasting memory for each shared library. But memory wastage
even worse when program
requesting memory with size more than 224KB(threshold value).

->If a program requests 1GB per request, it can use 45GB at the most.
->If a program requests 512MB per request, it can use 41.5GB at the most.
->If a program requests 225KB per request, it can use about 167MB at the
most.

As we ported  musl-1.1.14 for our architecture, we are bounded to make
change in same base code.
we have increased  MMAP_THRESHOLD to 1GB and also changes the calculation
for bin index .
after that observed improvement in memory utilization. i.e for size 225KB
memory used is 47.6 GB.

But now facing problem in multi threaded application. As we haven't changed
the function pretrim()
because there are some hard coded values like '40' and '3' used and unable
to understand how
these values are decided ..?

static int pretrim(struct chunk *self, size_t n, int i, int j)
{
        size_t n1;
        struct chunk *next, *split;

        /* We cannot pretrim if it would require re-binning. */
        if (j < 40) return 0;
        if (j < i+3) {
                if (j != 63) return 0;
                n1 = CHUNK_SIZE(self);
                if (n1-n <= MMAP_THRESHOLD) return 0;
        } else {
                n1 = CHUNK_SIZE(self);
        }
 .....
 .....
......
}

can we get any clue how these value are decided, it will be very
helpful for us.

Best Regard,
Ritesh Sonawane

On Tue, Jul 3, 2018 at 8:13 PM, Rich Felker <dalias@libc.org> wrote:

> On Tue, Jul 03, 2018 at 12:58:04PM +0530, ritesh sonawane wrote:
> > Hi All,
> >
> > We are using musl-1.1.14 version for our architecture. It is having page
> > size of 2MB.
> > Due to low threshold value there is more memory wastage. So we want to
> > change the value of  MMAP_THRESHOLD.
> >
> > can anyone please giude us, which factor need to consider to change this
> > threshold value ?
>
> It's not a parameter that can be changed but linked to the scale of
> bin sizes. There is no framework to track and reuse freed chunks which
> are larger than MMAP_THRESHOLD, so you'd be replacing recoverable
> waste from page granularity with unrecoverable waste from inability to
> reuse these larger freed chunks except breaking them up into pieces to
> satisfy smaller requests.
>
> I may look into handling this better when replacing musl's malloc at
> some point, but if your system forces you to use ridiculously large
> pages like 2MB, you've basically committed to wasting huge amounts of
> memory anyway (at least 2MB for each shared library in each
> process)...
>
> With musl git-master and future releases, you have the option to link
> a malloc replacement library that might be a decent solution to your
> problem.
>
> Rich
>

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