Re: [Qemu-devel] [RFC PATCH v6 3/3] Force auto-convegence of live migration

[Paolo Bonzini]

Il 14/06/2013 15:58, Chegu Vinod ha scritto:
> If a user chooses to turn on the auto-converge migration capability
> these changes detect the lack of convergence and throttle down the
> guest. i.e. force the VCPUs out of the guest for some duration
> and let the migration thread catchup and help converge.

Hi Vinod,

pretty much the same comments I sent you yesterday on the obsolete
version of the patch still apply.

> Verified the convergence using the following :
>  - Java Warehouse workload running on a 20VCPU/256G guest(~80% busy)
>  - OLTP like workload running on a 80VCPU/512G guest (~80% busy)
> 
> Sample results with Java warehouse workload : (migrate speed set to 20Gb and
> migrate downtime set to 4seconds).
> 
>  (qemu) info migrate
>  capabilities: xbzrle: off auto-converge: off  <----
>  Migration status: active
>  total time: 1487503 milliseconds
>  expected downtime: 519 milliseconds
>  transferred ram: 383749347 kbytes
>  remaining ram: 2753372 kbytes
>  total ram: 268444224 kbytes
>  duplicate: 65461532 pages
>  skipped: 64901568 pages
>  normal: 95750218 pages
>  normal bytes: 383000872 kbytes
>  dirty pages rate: 67551 pages
> 
>  ---
> 
>  (qemu) info migrate
>  capabilities: xbzrle: off auto-converge: on   <----
>  Migration status: completed
>  total time: 241161 milliseconds
>  downtime: 6373 milliseconds
>  transferred ram: 28235307 kbytes
>  remaining ram: 0 kbytes
>  total ram: 268444224 kbytes
>  duplicate: 64946416 pages
>  skipped: 64903523 pages
>  normal: 7044971 pages
>  normal bytes: 28179884 kbytes
> 
> Signed-off-by: Chegu Vinod <address@hidden>
> ---
>  arch_init.c |   85 
> +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
>  1 files changed, 85 insertions(+), 0 deletions(-)
> 
> diff --git a/arch_init.c b/arch_init.c
> index 5d32ecf..69c6c8c 100644
> --- a/arch_init.c
> +++ b/arch_init.c
> @@ -104,6 +104,8 @@ int graphic_depth = 15;
>  #endif
>  
>  const uint32_t arch_type = QEMU_ARCH;
> +static bool mig_throttle_on;
> +static void throttle_down_guest_to_converge(void);
>  
>  /***********************************************************/
>  /* ram save/restore */
> @@ -378,8 +380,15 @@ static void migration_bitmap_sync(void)
>      uint64_t num_dirty_pages_init = migration_dirty_pages;
>      MigrationState *s = migrate_get_current();
>      static int64_t start_time;
> +    static int64_t bytes_xfer_prev;
>      static int64_t num_dirty_pages_period;
>      int64_t end_time;
> +    int64_t bytes_xfer_now;
> +    static int dirty_rate_high_cnt;
> +
> +    if (!bytes_xfer_prev) {
> +        bytes_xfer_prev = ram_bytes_transferred();
> +    }
>  
>      if (!start_time) {
>          start_time = qemu_get_clock_ms(rt_clock);
> @@ -404,6 +413,23 @@ static void migration_bitmap_sync(void)
>  
>      /* more than 1 second = 1000 millisecons */
>      if (end_time > start_time + 1000) {
> +        if (migrate_auto_converge()) {
> +            /* The following detection logic can be refined later. For now:
> +               Check to see if the dirtied bytes is 50% more than the approx.
> +               amount of bytes that just got transferred since the last time 
> we
> +               were in this routine. If that happens >N times (for now N==4)
> +               we turn on the throttle down logic */
> +            bytes_xfer_now = ram_bytes_transferred();
> +            if (s->dirty_pages_rate &&
> +                ((num_dirty_pages_period*TARGET_PAGE_SIZE) >
> +                ((bytes_xfer_now - bytes_xfer_prev)/2))) {
> +                if (dirty_rate_high_cnt++ > 4) {

Too many parentheses, and please remove the nested if.

> +                    DPRINTF("Unable to converge. Throtting down guest\n");

Please use tracepoint instead.

> +                    mig_throttle_on = true;

Need to reset dirty_rate_high_cnt here, and both
dirty_rate_high_cnt/mig_throttle_on if you see !migrate_auto_converge().
 This ensures that throttling does not kick in automatically if you
disable and re-enable the feature.  It also lets you remove a bunch of
migrate_auto_converge() checks.

You also need to reset dirty_rate_high_cnt/mig_throttle_on in the setup
phase of migration.

> +                }
> +             }
> +             bytes_xfer_prev = bytes_xfer_now;
> +        }
>          s->dirty_pages_rate = num_dirty_pages_period * 1000
>              / (end_time - start_time);
>          s->dirty_bytes_rate = s->dirty_pages_rate * TARGET_PAGE_SIZE;
> @@ -628,6 +654,7 @@ static int ram_save_iterate(QEMUFile *f, void *opaque)
>          }
>          total_sent += bytes_sent;
>          acct_info.iterations++;
> +        throttle_down_guest_to_converge();

You can use a shorter name, like check_cpu_throttling().

>          /* we want to check in the 1st loop, just in case it was the 1st time
>             and we had to sync the dirty bitmap.
>             qemu_get_clock_ns() is a bit expensive, so we only check each some
> @@ -1098,3 +1125,61 @@ TargetInfo *qmp_query_target(Error **errp)
>  
>      return info;
>  }
> +
> +static bool throttling_needed(void)
> +{
> +    if (!migrate_auto_converge()) {
> +        return false;
> +    }
> +    return mig_throttle_on;
> +}
> +
> +/* Stub function that's gets run on the vcpu when its brought out of the
> +   VM to run inside qemu via async_run_on_cpu()*/
> +static void mig_sleep_cpu(void *opq)
> +{
> +    qemu_mutex_unlock_iothread();
> +    g_usleep(30*1000);
> +    qemu_mutex_lock_iothread();

Letting the user specify the entity of the pause would be nice, so that
management can ramp it up.  It can be done as a follow-up by adding a
'*value': 'int' field to MigrationCapabilityStatus (between 0 and 100,
default 30 as above).

Paolo

> +}
> +
> +/* To reduce the dirty rate explicitly disallow the VCPUs from spending
> +   much time in the VM. The migration thread will try to catchup.
> +   Workload will experience a performance drop.
> +*/
> +static void mig_throttle_cpu_down(CPUState *cpu, void *data)
> +{
> +    async_run_on_cpu(cpu, mig_sleep_cpu, NULL);
> +}
> +
> +static void mig_throttle_guest_down(void)
> +{
> +    if (throttling_needed()) {

No need for this "if", it is done already in the caller.

> +        qemu_mutex_lock_iothread();
> +        qemu_for_each_cpu(mig_throttle_cpu_down, NULL);
> +        qemu_mutex_unlock_iothread();
> +    }
> +}
> +
> +static void throttle_down_guest_to_converge(void)
> +{
> +    static int64_t t0;
> +    int64_t        t1;
> +
> +    if (!throttling_needed()) {

With the above suggested changes, this can simply check mig_throttle_on.

> +        return;
> +    }
> +
> +    if (!t0)  {
> +        t0 = qemu_get_clock_ns(rt_clock);
> +        return;
> +    }
> +
> +    t1 = qemu_get_clock_ns(rt_clock);
> +
> +    /* If it has been more than 40 ms since the last time the guest
> +     * was throtled then do it again.
> +     */

throttled

> +    if (((t1-t0)/1000000) > 40) {

I prefer moving the multiplication to the right so you don't need
parentheses, but this is _really_ a nit...

Paolo

> +        mig_throttle_guest_down();
> +        t0 = t1;
> +    }
> +}
>