m_tag, malloc vs uma

Robert Watson rwatson at FreeBSD.org
Fri Apr 10 13:02:44 PDT 2009

On Fri, 10 Apr 2009, Karim Fodil-Lemelin wrote:

> Thank you for the answer, clear and concise. I asked the question because I 
> had modified pf_get_mtag() to use uma directly in the hope that it would be 
> faster then calling malloc. But since pf_mtag is 20bytes, malloc will end up 
> using a fixed 32bytes zone and I shouldn't expect much speed gain from using 
> something like (except some savings from not having to select the 32bytes 
> zone):

There is another small overhead, the critical section used to protect the 
consistency of the per-CPU malloc type alloc and free counters, but it's also 
very small.

I think it would be desirable to make a change to more flexible m_tag types 
for 8.0, but I'm not sure I have time to implement/test it.  Is this something 
you might be interested in working on?  I'm thinking of basically replacing 
the m_tag_free pointer with a pointer to a small vector of operations, 
possibly something along these lines:

struct m_tag_ops {
 	void		(*m_tag_free)(struct m_tag *);
 	struct m_tag	(*m_tag_copy)(struct m_tag *);

If the m_tag_ops pointer is NULL, we go with today's default (requiring 
minimal change of existing consumers).  I'm not sure if there are any other 
function pointers we'd need at this point?

Robert N M Watson
Computer Laboratory
University of Cambridge

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