Re: [win-pv-devel] Very slow shared memory (gnttab)

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: win-pv-devel [mailto:win-pv-devel-bounces@xxxxxxxxxxxxxxxxxxxx] On
> Behalf Of Martin Cerveny
> Sent: 11 October 2016 14:50
> To: win-pv-devel@xxxxxxxxxxxxxxxxxxxx
> Subject: [win-pv-devel] Very slow shared memory (gnttab)
> 
> Hello.
> 
> I am facing very slow sharable memory (memcpy()) from Windows Guest
> (linux guests are Ok).
> - write to sharable memory ~ 2MB/s (NOT GB)
> - read from sharable memory ~ 4MB/s (NOT GB)
> - r/w from/to local memory ~ > 8GB/s
>

My guess would be that, because grants are mapped in the 'hole' which is the 
platform PCI device's BAR space, that the memory accesses are uncached, and so 
are very very slow.

  Paul
 
> - Dom0 shared 2048 refs (8MB RAM) (xc_gntshr_share_pages())
> - window7 x64 imported
> (IOCTL_XENIFACE_GNTTAB_MAP_FOREIGN_PAGES)
> - memcpy() to copy ~ 2MB data
> 
> - XenIface 8.2.0.51 (12.8.2016)
> - XenBus 8.2.0.125 (21.9.2016)
> - Windows 7SP1 + all patches
> - VS2013
> - Xen4.5.3, Xen4.6.1, Xen4.7.0
> - Dom0 kernel XenServer kernel-3.10.41-332.380424 or kernel-3.10.96-
> 484.383030
> 
> What is wrong ?
> Are there any tunables ?
> 
> Thanks for answer, Martin Cerveny
> 
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