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Date: Tue, 20 Jun 2017 22:04:45 +0200
From: Markus Wichmann <>
Subject: Re: Query regarding malloc if statement

On Tue, Jun 20, 2017 at 04:35:49PM +0200, Szabolcs Nagy wrote:
> are you arguing with somebody off-list?
> (i only see your replies)

I had a feeling something strange was going on lately. However, Jamie
did address his mails to the list. Could someone investigate where they
got stuck? Is he in your killfile, perchance?

> in any case the calloc code should not be
> controversial on linux.

It isn't, but valgrind likes to disagree (that's why I alluded to
reclaim() before; valgrind has a habit of playing contrarian to musl).

> writing to each allocated page when they are
> potentially unused is a huge waste of memory
> not just time. (a benchmark that calls calloc/free
> in a loop is obviously bogous, try
> calloc(1000*1000*1000,1)
> vs
> memset(malloc(1000*1000*1000),0,1000*1000*1000))

Well, that doesn't test the line in contention as the allocated size is
way beyond the mmap threshold; thus __malloc0() won't do anything at
all, except call malloc().

The only way that line would make a positive difference is if malloc()
were to call expand_heap(), and expand_heap() were to use mmap() to get
more memory, and the returned chunk would contain unallocated pages.
Since each heap expansion requires the writing of a header and a footer,
something between three pages and the mmap threshold would have to be
allocated. That's why I suggested using 100 kB.


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