malloc(9) and its alignment

[Gumpula, Suresh]

Thanks for the explanation John.  How about porting ARM way of handling required alignment with creation of separate zones
And allocating with uma_zalloc(zone) for  AMD64 too  for bus_dmamem_alloc?

Thanks
Suresh


-----Original Message-----
From: John-Mark Gurney [mailto:jmg at funkthat.com] 
Sent: Wednesday, February 12, 2014 5:07 PM
To: Gumpula, Suresh
Cc: Ian Lepore; freebsd-hackers at freebsd.org
Subject: Re: malloc(9) and its alignment

Gumpula, Suresh wrote this message on Wed, Feb 12, 2014 at 19:40 +0000:
> Thanks Ian for the reply.   I will look at the ARM code, but I was  thinking  why malloc(9) does not return bucket size aligned pointers. 

Always returning bucket sizes aligned pointers may not be ideal for a cache.. say you have a buffer of 512 bytes, where often only the first
128 bytes are used (but all 512 bytes may be)...  If you always align at 512, some cache lines will be more heavily used than others, reducing the effective size of the cache...

This is only one reason not aligning to size may be better...

-- 
  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."