Le 04/06/2012 11:35, Alexandre DERUMIER a écrit :
Hi,
about this:
Turning off Virtualisation extension in BIOS. Don't know why, but it
gaves us crappy performance. We usually put it on, because we use KVM a
lot. In our case, OSD are in bare metal and disabling virtualisation
extension gives us a very big boost.
It may be a BIOS bug in our machines (DELL M610).
It could be related to iommu, if you pass intel_iommu=on in grub.
I have already had this kind of problem.
When intel_iommu=on, Linux (completely unrelated to KVM) adds a new level
of protection which didn't exist without an IOMMU - the network card, which
without an IOMMU could write (via DMA) to any memory location, now is
not allowed - the card can only write to memory locates which the OS
wanted it to write. Theoretically, this can protect the OS against
various kinds of attacks. But what happens now is that every time that
Linux passes a new buffer to the card, it needs to change the IOMMU
mappings. This noticably slows down I/O, unfortunately.
Infortunately, this is not the case. The intel card supports it, but
DELL M160 don't.
And I just checked, ou linux command line don't include intel_iommu=on.
BTW, it seems that turning on virtualization on bios kills performance
on integrated ixgbe driver. Sourceforge one seems less affected. Our
tests were circa kernel 3.2 , it may have changed since.
Cheers,
--
Yann Dupont - Service IRTS, DSI Université de Nantes
Tel : 02.53.48.49.20 - Mail/Jabber : Yann.Dupont@xxxxxxxxxxxxxx
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