Re: [Xen-devel] Signed GPLPV drivers available for download

[James Harper <james.harper@xxxxxxxxxxxxxxxx>]

> 
> glad you ask - because just yesterday i re-tested the 357-version of the
> drivers under Windows7 64bit HVM guest - here are the results:
> 
> (everyting in mb/s)
> 
> W/O gplpv-drivers and WITH pci-passthrough:
> 
> Seq. reading: 311.8
> Seq. writing:  106.3
> 4k    reading: 10.2
> 4k    writing:   9.3
> 4k64thread reading: 11.1
> 4k64threads writing: 10.9
> 
> 
> WITH gplpv-drivers and WITH pci-passthrough:
> 
> Seq. reading: 98.8
> Seq. writing:  31.6
> 4k    reading: 12.0
> 4k    writing:  19.3
> 4k64thread reading: 15.4
> 4k64threads writing: 29.6
> 
> 
> WITH gplpv-drivers and WITHOUT pci-passthrough:
> 
> Seq. reading: 104.4
> Seq. writing:   85.2
> 4k    reading:  12.7
> 4k    writing:   20.4
> 4k64thread reading: 16.6
> 4k64threads writing: 32.6
> 
> 
> Strange thing is, that even without pci-passthrough the performance with
> gplpv is'nt that much better compared to w/o gplpv but with pci-passthrough
> - well it is overall a bit better, but just a bit, and seq. read performance 
> is
> much worse...
> i haven't made a test without gplpv and without passthrough at the same
> time - tell me if you want to see how that performs.
> 
> Greetings!
> Tobias
> 
> P.S.: i'm using phy backend pointing to a LVM device on an SSD for this tests.
> 

What are you testing this with? I just ran some tests with iometer under 2008R2 
and it reminded me of something... DRBD *hates* having outstanding writes to 
the same block, and complains about it, so gplpv serialises such requests 
(stalling the queue until the first request is complete). This almost never 
happens in production but iometer sends such requests frequently, which will 
significantly impact performance.

You can test with the debug build to see if this is happening with whatever you 
are testing with - you'll see messages like "Concurrent outstanding write 
detected". Be aware though that qemu will rate limit writes to the debug log so 
if it happens a lot (like under iometer) it will slow to a crawl.

James


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