Re: Changing MMAP_THRESHOLD in malloc() implementation.

[Rich Felker <dalias@libc.org>]

On Wed, Jul 04, 2018 at 10:54:50AM +0530, ritesh sonawane wrote:
> 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.

How does this happen? The behavior you should see is just rounding up
of the request to a multiple of the page size, not scaling of the
request. Maybe I don't understand what you're saying here.

> 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.

You can just disable use of pretrim, at a performance cost -- it's an
optimization to avoid shuffling in and out of bins when the remainder
will go back in the same bin.

Unfortunately you're right that the magic numbers in pretrim aren't
immediately clear in their purpose so I'd have to take some time to
look at what they're doing. Some of the checks might be redundant
(just a short circuit to save time).

Rich