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Message-ID: <20170620200445.brsuiy6ssojslmau@voyager>
Date: Tue, 20 Jun 2017 22:04:45 +0200
From: Markus Wichmann <nullplan@....net>
To: musl@...ts.openwall.com
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.
Ciao,
Markus
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