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70,000 TLB shootdown IPIs per second

I have been doing some testing on a fileserver recently donated to TNF.

The system has an Areca 1280 controller (arcmsr driver) with a single
RAID6 volume configured on 12 disks; 32GB of RAM; two quad-core Xeon L5420

I have tested under NetBSD-6 and NetBSD-current as well as the tls-maxphys
branch.  The test is a 'dd bs=2m of=/test count=65536".  Write throughput,
while acceptable (300-350MB/sec; 350-400MB/sec with tls-maxphys) is about
2/3 what I get on the same hardware under Linux.  Read throughput is less
good, 250-300MB/sec.

The filesystem is FFSv2, 32k block/4k frag, with WAPBL.

Watching systat while I do the dd tests, I see up to 70,000 TLB shootdown
IPIs per second.  Is this really right?  I am not sure I know how to count
these under Linux but I don't see any evidence of them.  Is there a pmap
problem?  I'm running port-amd64.

I have also seen some other odd things I'll detail elsewhere, but just for
a start, can anyone explain to me whether this number of TLB shootdowns
should be expected, whether each should really generate its own IPI, and
what the performance impact may be?

 Thor Lancelot Simon                                          

        It's very complicated.  It's very cumbersome.  There's a
        lot of numbers involved with it.

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