Re: [Xen-devel] PHY vs. VD benchmark

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

> I suspect you might be able to increase the performance by using a different 
> extent size.  The VD management code allocates extents of "initialised" 
> physical disks.  The extent size for a device can be specified when it's 
> initialised.  The default is 64 megabytes.  A 4 gigabyte virtual disk would 
> make for 64 extents of 64 meg.  A long list of extents could be slowing down 
> the disk address translation in Xen, resulting in poorer benchmark results.  
> At some stage I might rethink the default extent size.

The correct fix is to get rid of the linear linked lists.

A good way to do this is to use a buddy allocator for allocating VDs:
you track power-of-two multiples of 64MB free disc space, and try to
create VDs out of the largest possible extents. Within Xen you can
then use a truncated radix tree to map virtual extents to real extents
--- Linux has code we can use for truncated radix trees.

 -- Keir

> If you want, you could try a larger extent size by specifying it at 
> initialisation time.  [ you can't change the extent size on an already 
> initialised device :-( ].  This might speed things up in the benchmark, 
> although if you don't allocate space in multiples of the extent size there 
> will be more space in unused "partial extents" than for a smaller extent size.
> 
> In the degenerate case that your entire virtual disk fits inside an extent, 
> it 
> should give exactly the same performance as for the phy case (since the code 
> will be doing exactly the same thing).
> 
> If I get some spare time I might test this out myself - right now I'm 
> debugging a different problem.
> 
> Cheers,
> 
> Mark
> 
> 
> 
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