Re: [Qemu-devel] [PATCH 2/2] migration: Dynamic cpu throttling for auto-converge

[Dr. David Alan Gilbert]

* Jason J. Herne (address@hidden) wrote:
> On 06/02/2015 09:58 AM, Dr. David Alan Gilbert wrote:
> >* Jason J. Herne (address@hidden) wrote:
> >>On 06/01/2015 11:32 AM, Dr. David Alan Gilbert wrote:
> >>>* Jason J. Herne (address@hidden) wrote:
> >>>>Remove traditional auto-converge static 30ms throttling code and replace 
> >>>>it
> >>>>with a dynamic throttling algorithm.
> >>>>
> >>>>Additionally, be more aggressive when deciding when to start throttling.
> >>>>Previously we waited until four unproductive memory passes. Now we begin
> >>>>throttling after only two unproductive memory passes. Four seemed quite
> >>>>arbitrary and only waiting for two passes allows us to complete the 
> >>>>migration
> >>>>faster.
> >>>>
> >>>>Signed-off-by: Jason J. Herne <address@hidden>
> >>>>Reviewed-by: Matthew Rosato <address@hidden>
> >>>>---
> >>>>  arch_init.c           | 95 
> >>>> +++++++++++++++++----------------------------------
> >>>>  migration/migration.c |  9 +++++
> >>>>  2 files changed, 41 insertions(+), 63 deletions(-)
> >>>>
> >>>>diff --git a/arch_init.c b/arch_init.c
> >>>>index 23d3feb..73ae494 100644
> >>>>--- a/arch_init.c
> >>>>+++ b/arch_init.c
> >>>>@@ -111,9 +111,7 @@ int graphic_depth = 32;
> >>>>  #endif
> >>>>
> >>>>  const uint32_t arch_type = QEMU_ARCH;
> >>>>-static bool mig_throttle_on;
> >>>>  static int dirty_rate_high_cnt;
> >>>>-static void check_guest_throttling(void);
> >>>>
> >>>>  static uint64_t bitmap_sync_count;
> >>>>
> >>>>@@ -487,6 +485,31 @@ static size_t save_page_header(QEMUFile *f, RAMBlock 
> >>>>*block, ram_addr_t offset)
> >>>>      return size;
> >>>>  }
> >>>>
> >>>>+/* Reduce amount of guest cpu execution to hopefully slow down memory 
> >>>>writes.
> >>>>+ * If guest dirty memory rate is reduced below the rate at which we can
> >>>>+ * transfer pages to the destination then we should be able to complete
> >>>>+ * migration. Some workloads dirty memory way too fast and will not 
> >>>>effectively
> >>>>+ * converge, even with auto-converge. For these workloads we will 
> >>>>continue to
> >>>>+ * increase throttling until the guest is paused long enough to complete 
> >>>>the
> >>>>+ * migration. This essentially becomes a non-live migration.
> >>>>+ */
> >>>>+static void mig_throttle_guest_down(void)
> >>>>+{
> >>>>+    CPUState *cpu;
> >>>>+
> >>>>+    CPU_FOREACH(cpu) {
> >>>>+        /* We have not started throttling yet. Lets start it.*/
> >>>>+        if (!cpu_throttle_active(cpu)) {
> >>>>+            cpu_throttle_start(cpu, 0.2);
> >>>>+        }
> >>>>+
> >>>>+        /* Throttling is already in place. Just increase the throttling 
> >>>>rate */
> >>>>+        else {
> >>>>+            cpu_throttle_start(cpu, cpu_throttle_get_ratio(cpu) * 2);
> >>>>+        }
> >>>
> >>>Now that migration has migrate_parameters, it would be best to replace
> >>>the magic numbers (the 0.2, the *2 - anything else?)  by parameters that 
> >>>can
> >>>change the starting throttling and increase rate.  It would probably also 
> >>>be
> >>>good to make the current throttling rate visible in info somewhere; maybe
> >>>info migrate?
> >>>
> >>
> >>I did consider all of this. However, I don't think that the controls
> >>this patch provides are an ideal throttling mechanism. I suspect someone
> >>with
> >>vcpu/scheduling experience could whip up something more user friendly and
> >>cleaner.
> >>I merely propose this because it seems better than what we have today for
> >>auto-converge.
> >>
> >>Also, I'm not sure how useful the information really is to the user. The
> >>fact that it is a ratio instead of a percentage might be confusing. Also,
> >>I suspect it is not
> >>truly very accurate. Again, I was going for "make it better", not "make it
> >>perfect".
> >>
> >>Lastly, if we define this external interface we are kind of stuck with it,
> >>yes?
> >
> >Well, one thing you can do is add a parameter with a name starting with x-
> >which means it's not a fixed interface (so things like libvirt wont use it).
> >And the reason I was interested in seeing the information was otherwise
> >we don't really have any way of knowing how well the code is working;
> >is it already throttling down more and more?
> >
> 
> Fair point. Can we add a qmp/hmp command like "info x-cpu-throttle"? Or a
> new line in "info migrate" output, "x-throttle-ration:  1.0" perhaps?
> Would this mess up libvirt's parsing of the hmp/qmp data?

A friendly libvirt person said that it will ignore extra data
(and if it doesn't it's a bug).

Dave

> -- 
> -- Jason J. Herne (address@hidden)
> 
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK