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

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Jason J. Herne (jjherne@linux.vnet.ibm.com) wrote:
> On 06/01/2015 11:32 AM, Dr. David Alan Gilbert wrote:
> >* Jason J. Herne (jjherne@linux.vnet.ibm.com) 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 <jjherne@linux.vnet.ibm.com>
> >>Reviewed-by: Matthew Rosato <mjrosato@linux.vnet.ibm.com>
> >>---
> >>  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?

> In
> this regard we should be sure that this is how we want cpu throttling to
> work.  Alternatively, I propose to accept this patch set as-is and then work on a
> real vcpu Throttling mechanism that can be used for auto-converge as well as a
> user controllable guest throttling/limiting mechanism. Once that is in place we
> can migrate (no pun intended) the auto-converge code to the new way and remove
> this stuff.

Yes, it's probably still better than what we already have.

Dave


> 
> With all of that said, I'm willing to provide the requested controls if we
> really
> feel the pros outweigh the cons. Thanks for your review :).
> 
> ...
> 
> -- 
> -- Jason J. Herne (jjherne@linux.vnet.ibm.com)
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK