VM_BCACHE_SIZE_MAX on i386

[Konstantin Belousov]

On Sat, Mar 23, 2013 at 04:49:06PM -0500, Alan Cox wrote:
> The additional KVA that we had to reserve for the vm_page radix tree nodes
> already got me thinking about VM_BCACHE_SIZE_MAX a couple weeks ago.  With
> the extra KVA pressure that is inherent to PAE, e.g., a larger vm_page
> struct, we really can't afford to allow the buffer map KVA allocation to
> grow much beyond what it would be for a 4GB machine anyway.  Moreover, your
> work makes the size of the buffer map less important, because it will see
> decreasing use as drivers are converted to allow unmapped I/O.  So, I would
> encourage you to simply use the same cap based on a 4 GB machine for both
> PAE and !PAE.

I did not checked it, but isn't default PAE config splits user/kernel
on the 2GB boundary ? This makes the KVA pressure on PAE less severe,
but user mode should be not very happy.

Currently, the number of the regular buffer headers allocated is equal
to the size of the buffer map / BKVASIZE still. This could be changed
now, I believe that Peter' testing fixed most of the bugs in the
handling of the maxbufspace. I was too coward to make this change
together with the rest of the work. But it could indeed be useful, since
buffer map is used only by the metadata buffers for UFS.

I did changed VM_BCACHE_SIZE_MAX as you suggested, thanks.

http://people.freebsd.org/~kib/misc/i386_maxbcache.2.patch
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