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Date:      Sun, 23 Nov 2008 21:46:08 -1000 (HST)
From:      Jeff Roberson <jroberson@jroberson.net>
To:        arch@freebsd.org
Subject:   Limiting mbuf memory.
Message-ID:  <20081123213232.A971@desktop>

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I'm developing a patch for an alternate memory layout for mbuf clusters 
that relies on contigmalloc.  Since this can fail, we'll still have to 
retain the capability of allocating traditional clusters.  I'll report 
details on that later.  I'm writing this email to address the issue of 
resource accounting in mbufs.

Presently we use a set of limits on individual zones or sizes of mbufs. 
Standard mbufs, clusters, page size jumbos, 9k jumbos, and 16k jumbos. 
Each is administered sperately.  I think this is getting a bit unwieldy. 
In the future, we may have even more sizes.  This also introduces problems 
because I will have two cluster zones do they each get their own limit?

I would like to consolidate this into a single limit on the number of 
pages in total allocated to networking.  With perhaps some fractional 
reservation for standard mbufs and clusters to make sure they aren't 
overwhelmed by the larger buffers.

This would be implemented by overriding the uma zone page allocator for 
each of the mbuf zones with one that counts pages for all.  Should we 
reach the limit we'll block depending on the wait settings of the 
requestor.  The limit and sleep will probably be protected by a global 
lock which won't be an issue because trips to the back end allocator are 
infrequent and protected by their own global lock anyhow.

How do people feel about this?  To be clear this would eliminate:

nmbclusters, nmbjumbop, nmbjumbo9, nmbjumbo16 and related config settings 
and sysctls.  They would be replaced by something like 'maxmbufbytes'. 
Presently we place no limit on small mbufs.  I could go either way on 
this.  It could be added to the limit or not.

Thanks,
Jeff



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