Differences in malloc between 6 and 7?

Kris Kennaway kris at FreeBSD.org
Mon Mar 3 22:19:10 UTC 2008

Bakul Shah wrote:
> On Mon, 03 Mar 2008 15:23:33 EST gnn at freebsd.org  wrote:
>> One of the folks I'm working with found this.  The following code,
>> which yes, is just an example, is 1/2 as fast on 7.0-RELEASE as on
>> 6.3.  Where should I look to find out why?
> Specifying malloc option K (double virtual memory chunk size)
> roughly halves the runtime.  Additional Ks reduce it more and
> more.  As this test spends most of its time in the kernel,
> may be this is just due to the mmap overhead? Or may be the
> defaults for 6.3 were different.

Well, the whole architecture of malloc is different.

I also see a big performance cliff (drops by a factor of 10) when malloc 
size exceeds the chunk size (1MB by default).  Also concurrent access to 
mmapped memory performs badly in FreeBSD right now.  I have patches that 
convert the vm_map lock to a sx to avoid this contention, but they need 
some more work.


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