Re: [PATCH v3 3/3] tools: introduce parameter max_wp_ram_ranges.

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----Original Message-----
[snip]
> >>> Compare this to the downsides of the approach you're proposing:
> >>> 1. Using 40 bytes of hypervisor space per guest GPU pagetable page (as
> >>> opposed to using a bit in the existing p2m table)
> >>> 2. Walking down an RB tree with 8000 individual nodes to find out
> >>> which server to send the message to (rather than just reading the
> >>> value from the p2m table).
> >>
> >> 8K is an upper limit for the rangeset, in many cases the RB tree will
> >> not contain that many nodes.
> >>
> >>> 3. Needing to determine on a guest-by-guest basis whether to change
> the
> >> limit
> >>> 4. Needing to have an interface to make the limit even bigger, just in
> >>> case we find workloads that have even more GTTs.
> >>>
> >>
> >> Well, I have suggested in yesterday's reply. XenGT can choose not to
> >> change this limit even when workloads are getting heavy - with
> >> tradeoffs in the device model side.
> >
> > I assume this means that the emulator can 'unshadow' GTTs (I guess on an
> LRU basis) so that it can shadow new ones when the limit has been
> exhausted?
> > If so, how bad is performance likely to be if we live with a lower limit and
> take the hit of unshadowing if the guest GTTs become heavily fragmented?
> >
> Thank you, Paul.
> 
> Well, I was told the emulator have approaches to delay the shadowing of
> the GTT till future GPU commands are submitted. By now, I'm not sure
> about the performance penalties if the limit is set too low. Although
> we are confident 8K is a secure limit, it seems still too high to be
> accepted. We will perform more experiments with this new approach to
> find a balance between the lowest limit and the XenGT performance.
> 
> So another question is, if value of this limit really matters, will a
> lower one be more acceptable(the current 256 being not enough)?

Well, do you know conclusively that 256 is absolutely not enough, or is it just too low for acceptable performance?

  Paul

> 
> Thanks
> Yu
> find a