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Re: [Xen-devel] [PATCH] full support of setting scheduler parameters on domain creation

  • To: Ian Campbell <Ian.Campbell@xxxxxxxxxx>
  • From: Juergen Gross <juergen.gross@xxxxxxxxxxxxxx>
  • Date: Mon, 21 May 2012 15:48:15 +0200
  • Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Mon, 21 May 2012 13:48:45 +0000
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  • List-id: Xen developer discussion <xen-devel.lists.xen.org>

On 05/21/2012 03:34 PM, Ian Campbell wrote:
On Mon, 2012-05-21 at 14:23 +0100, Juergen Gross wrote:
On 05/21/2012 02:07 PM, Ian Campbell wrote:
On Mon, 2012-05-21 at 12:46 +0100, Juergen Gross wrote:
Obtains current scheduler parameters before modifying any settings specified
via domain config. Only specified settings must be modified, of course!

I presume this will fix the
libxl: error: libxl.c:3208:libxl_sched_credit_domain_set: Cpu weight out
of range, valid values are within range from 1 to 65535
warning we are currently seeing? Thanks!

@@ -233,6 +233,14 @@ libxl_sched_params = Struct("sched_param
       ("slice",        integer),
       ("latency",      integer),
       ("extratime",    integer),
+    ("set_weight",       bool),
+    ("set_cap",          bool),
+    ("set_tslice_ms",    bool),
+    ("set_ratelimit_us", bool),
+    ("set_period",       bool),
+    ("set_slice",        bool),
+    ("set_latency",      bool),
+    ("set_extratime",    bool),
Rather than doing this it would be preferable to identify some specific
value which means "default" for each of these fields. Generally this
would be either 0 (preferred if possible) or ~0 or -1. You can then
describe this in the IDL using the "init_val" property on each field.

@@ -225,7 +225,7 @@ libxl_domain_create_info = Struct("domai
   MemKB = UInt(64, init_val = "LIBXL_MEMKB_DEFAULT")

   libxl_sched_params = Struct("sched_params",[
-    ("weight",       integer),
+    ("weight",       integer, {'init_val': -1}),
       ("cap",          integer),
       ("tslice_ms",    integer),
       ("ratelimit_us", integer),

(just as an illustration of a non-zero default, I suspect 0 would
actually be a fine default value for weight, and 0 is the default

Then libxl_sched_params_init, which xl must call (perhaps indirectly,
e.g. via libxl_domain_build_info_init) would set these defaults and xl
would override them from the config and libxl would only set those which
were non-default.
Hmm. Scheduling parameters are handled in the hypervisor. I don't want to
export the knowledge about semantics to the tools. If this is no problem,
why can't I just set the defaults in the tools and omit asking the
hypervisor for the current settings?
Exporting the idea that 0 is not a valid weight is (IMHO at least)
better than exporting the fact that the default weight is (e.g.) 200 and
hard coding that in multiple places.


Additionally there are parameters (well, one parameter) which apply to
multiple schedulers. Just by chance -1 would be invalid for all of them,
but I don't like to hard code this coincidence.
Which parameter is it? Is there any reasonable possibility that a
scheduler would ever use -1 (or +4 billion) for it?

It's the cpu_weight. :-)
I don't object using an invalid value instead of a boolean, I just thought
it would be cleaner to say "parameter xy was specified". This would include
the case when a user specifies 0 for the cpu_weight: it would be rejected
instead of just ignored...

You could define a symbolic name if that would make you more comfortable
(that would allow us to change the specific value without changing the

As the "invalid" value would be used at least twice this is a must.


Juergen Gross                 Principal Developer Operating Systems
PDG ES&S SWE OS6                       Telephone: +49 (0) 89 3222 2967
Fujitsu Technology Solutions              e-mail: juergen.gross@xxxxxxxxxxxxxx
Domagkstr. 28                           Internet: ts.fujitsu.com
D-80807 Muenchen                 Company details: ts.fujitsu.com/imprint.html

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