* Jason J. Herne (jjhe...@linux.vnet.ibm.com) wrote:
> On 06/01/2015 11:32 AM, Dr. David Alan Gilbert wrote:
> >* Jason J. Herne (jjhe...@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 <jjhe...@linux.vnet.ibm.com>
> >>Reviewed-by: Matthew Rosato <mjros...@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 (jjhe...@linux.vnet.ibm.com)
>
--
Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK
