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[Xen-devel] Small address spaces update

To: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: [Xen-devel] Small address spaces update
From: Jacob Gorm Hansen <jacobg@xxxxxxx>
Date: Mon, 25 Apr 2005 20:41:45 -0700
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hi,

I have managed to get the 'small address spaces' hack running in xen-unstable. I have relinked my dom0 userspace (busybox + vmtools) and kernel to share the range 0xf0000000 - 0xfc000000, and my domU kernel to stay below 0xf0000000, on x86-32.

I use the top part of the domU page directories as a cache for the currently running I/O-domain (right now I only have one, dom0), and so unless I need to update this cache I can context switch without flushing the TLB. This saves more 99% af TLB flushes when doing simple I/O-bound stuff such as running iPerf inside a domU.

In some cases (when dom0 has updated or switched its page directory) I need to update the cache in the domU page directory, that involves doing a memcpy of 48*4 = 192 bytes. How frequently this happens depends on the kind of stuff running in dom0 userspace, i.e. for the iperf test is very rare.

At this point I have no idea if this increases domU I/O performance, as right now I am getting bad iperf numbers in domU both with and without this patch applied to Xen, but just wanted to let people know that I am still working on this thing when I can find the time.

Best regards,
Jacob

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