Re: [Qemu-devel] [RFC PATCH v2] Throttle-down guest when live migration does not converge.

[Orit Wasserman]

On 04/27/2013 11:50 PM, Chegu Vinod wrote:
> Busy enterprise workloads hosted on large sized VM's tend to dirty
> memory faster than the transfer rate achieved via live guest migration.
> Despite some good recent improvements (& using dedicated 10Gig NICs
> between hosts) the live migration does NOT converge.
> 
> A few options that were discussed/being-pursued to help with
> the convergence issue include:
> 
> 1) Slow down guest considerably via cgroup's CPU controls - requires
>    libvirt client support to detect & trigger action, but conceptually
>    similar to this RFC change.
> 
> 2) Speed up transfer rate:
>    - RDMA based Pre-copy - lower overhead and fast (Unfortunately
>      has a few restrictions and some customers still choose not
>      to deploy RDMA :-( ).
>    - Add parallelism to improve transfer rate and use multiple 10Gig
>      connections (bonded). - could add some overhead on the host.
> 
> 3) Post-copy (preferably with RDMA) or a Pre+Post copy hybrid - Sounds
>    promising but need to consider & handle newer failure scenarios.
> 
> If an enterprise user chooses to force convergence of their migration
> via the new capability "auto-converge" then with this change we auto-detect
> lack of convergence scenario and trigger a slow down of the workload
> by explicitly disallowing the VCPUs from spending much time in the VM
> context.
> 
> The migration thread tries to catchup and this eventually leads
> to convergence in some "deterministic" amount of time. Yes it does
> impact the performance of all the VCPUs but in my observation that
> lasts only for a short duration of time. i.e. we end up entering
> stage 3 (downtime phase) soon after that.
> 
> No exernal trigger is required (unlike option 1) and it can co-exist
> with enhancements being pursued as part of Option 2 (e.g. RDMA).
> 
> Thanks to Juan and Paolo for their useful suggestions.
> 
> Verified the convergence using the following :
> - SpecJbb2005 workload running on a 20VCPU/256G guest(~80% busy)
> - OLTP like workload running on a 80VCPU/512G guest (~80% busy)
> 
> Sample results with SpecJbb2005 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
> 
> Changes from v1:
> - rebased to latest qemu.git
> - added auto-converge capability(default off) - suggested by Anthony Liguori &
>                                                 Eric Blake.
> 
> Signed-off-by: Chegu Vinod <address@hidden>
> ---
>  arch_init.c                   |   44 +++++++++++++++++++++++++++++++++++
>  cpus.c                        |   12 +++++++++
>  include/migration/migration.h |   12 +++++++++
>  include/qemu/main-loop.h      |    3 ++
>  kvm-all.c                     |   51 
> +++++++++++++++++++++++++++++++++++++++++
>  migration.c                   |   15 ++++++++++++
>  qapi-schema.json              |    6 ++++-
>  7 files changed, 142 insertions(+), 1 deletions(-)
> 
> diff --git a/arch_init.c b/arch_init.c
> index 92de1bd..6dcc742 100644
> --- a/arch_init.c
> +++ b/arch_init.c
> @@ -104,6 +104,7 @@ int graphic_depth = 15;
>  #endif
>  
>  const uint32_t arch_type = QEMU_ARCH;
> +static uint64_t mig_throttle_on;
>  
>  /***********************************************************/
>  /* ram save/restore */
> @@ -379,12 +380,20 @@ static void migration_bitmap_sync(void)
>      MigrationState *s = migrate_get_current();
>      static int64_t start_time;
>      static int64_t num_dirty_pages_period;
> +    static int64_t bytes_xfer_prev;
>      int64_t end_time;
> +    int64_t bytes_xfer_now;
> +    static int dirty_rate_high_cnt;
> +
> +    if (migrate_auto_converge() && !bytes_xfer_prev) {
> +        bytes_xfer_prev = ram_bytes_transferred();
> +    }
>  
>      if (!start_time) {
>          start_time = qemu_get_clock_ms(rt_clock);
>      }
>  
> +
>      trace_migration_bitmap_sync_start();
>      memory_global_sync_dirty_bitmap(get_system_memory());
>  
> @@ -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==5)
> +               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++ > 5) {
> +                    DPRINTF("Unable to converge. Throtting down guest\n");
> +                    mig_throttle_on = 1;
> +                }

Why not check to see if mig_throttle_on is already on?

> +             }

> +             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;
> @@ -496,6 +522,24 @@ static int ram_save_block(QEMUFile *f, bool last_stage)
>      return bytes_sent;
>  }
>  
> +bool throttling_needed(void)
> +{
> +    bool value;
> +
> +    if (!migrate_auto_converge()) {
> +        return false;
> +    }
> +
> +    qemu_mutex_lock_mig_throttle();
> +    value = mig_throttle_on;
> +    qemu_mutex_unlock_mig_throttle();
> +
> +    if (value) {
> +        return true;
> +    }
> +    return false;
> +}
> +

Why not return value here ?

Cheers,
Orit

>  static uint64_t bytes_transferred;
>  
>  static ram_addr_t ram_save_remaining(void)
> diff --git a/cpus.c b/cpus.c
> index 5a98a37..615c25a 100644
> --- a/cpus.c
> +++ b/cpus.c
> @@ -616,6 +616,7 @@ static void qemu_tcg_init_cpu_signals(void)
>  #endif /* _WIN32 */
>  
>  static QemuMutex qemu_global_mutex;
> +static QemuMutex qemu_mig_throttle_mutex;
>  static QemuCond qemu_io_proceeded_cond;
>  static bool iothread_requesting_mutex;
>  
> @@ -638,6 +639,7 @@ void qemu_init_cpu_loop(void)
>      qemu_cond_init(&qemu_work_cond);
>      qemu_cond_init(&qemu_io_proceeded_cond);
>      qemu_mutex_init(&qemu_global_mutex);
> +    qemu_mutex_init(&qemu_mig_throttle_mutex);
>  
>      qemu_thread_get_self(&io_thread);
>  }
> @@ -943,6 +945,16 @@ void qemu_mutex_unlock_iothread(void)
>      qemu_mutex_unlock(&qemu_global_mutex);
>  }
>  
> +void qemu_mutex_lock_mig_throttle(void)
> +{
> +    qemu_mutex_lock(&qemu_mig_throttle_mutex);
> +}
> +
> +void qemu_mutex_unlock_mig_throttle(void)
> +{
> +    qemu_mutex_unlock(&qemu_mig_throttle_mutex);
> +}
> +
>  static int all_vcpus_paused(void)
>  {
>      CPUArchState *penv = first_cpu;
> diff --git a/include/migration/migration.h b/include/migration/migration.h
> index e2acec6..94bdb8c 100644
> --- a/include/migration/migration.h
> +++ b/include/migration/migration.h
> @@ -127,4 +127,16 @@ int migrate_use_xbzrle(void);
>  int64_t migrate_xbzrle_cache_size(void);
>  
>  int64_t xbzrle_cache_resize(int64_t new_size);
> +
> +#ifndef _QEMU_MIG_THROTTLE
> +#define _QEMU_MIG_THROTTLE

Do you need those defines?

> +
> +bool migrate_auto_converge(void);
> +
> +bool throttling_needed(void);
> +bool throttling_now(void);
> +void *migration_throttle_down(void *);
> +
> +#endif
> +
>  #endif
> diff --git a/include/qemu/main-loop.h b/include/qemu/main-loop.h
> index 6f0200a..9a3886d 100644
> --- a/include/qemu/main-loop.h
> +++ b/include/qemu/main-loop.h
> @@ -299,6 +299,9 @@ void qemu_mutex_lock_iothread(void);
>   */
>  void qemu_mutex_unlock_iothread(void);
>  
> +void qemu_mutex_lock_mig_throttle(void);
> +void qemu_mutex_unlock_mig_throttle(void);
> +
>  /* internal interfaces */
>  
>  void qemu_fd_register(int fd);
> diff --git a/kvm-all.c b/kvm-all.c
> index 2d92721..a92cb77 100644
> --- a/kvm-all.c
> +++ b/kvm-all.c
> @@ -33,6 +33,8 @@
>  #include "exec/memory.h"
>  #include "exec/address-spaces.h"
>  #include "qemu/event_notifier.h"
> +#include "sysemu/cpus.h"
> +#include "migration/migration.h"
>  
>  /* This check must be after config-host.h is included */
>  #ifdef CONFIG_EVENTFD
> @@ -116,6 +118,8 @@ static const KVMCapabilityInfo kvm_required_capabilites[] 
> = {
>      KVM_CAP_LAST_INFO
>  };
>  
> +static void mig_delay_vcpu(void);
> +
>  static KVMSlot *kvm_alloc_slot(KVMState *s)
>  {
>      int i;
> @@ -1609,6 +1613,10 @@ int kvm_cpu_exec(CPUArchState *env)
>          }
>          qemu_mutex_unlock_iothread();
>  
> +        if (throttling_needed()) {
> +            mig_delay_vcpu();
> +        }
> +
>          run_ret = kvm_vcpu_ioctl(cpu, KVM_RUN, 0);
>  
>          qemu_mutex_lock_iothread();
> @@ -2032,3 +2040,46 @@ int kvm_on_sigbus(int code, void *addr)
>  {
>      return kvm_arch_on_sigbus(code, addr);
>  }
> +
> +static bool throttling;
> +bool throttling_now(void)
> +{
> +    if (throttling) {
> +        return true;
> +    }
> +    return false;
> +}
> +

it will be simpler to just return throttling ?

> +static void mig_delay_vcpu(void)
> +{
> +    g_usleep(50*1000);
> +}
> +
> +/* Stub used for getting the vcpu out of VM and into qemu via
> +   run_on_cpu()*/
> +static void mig_kick_cpu(void *opq)
> +{
> +    return;
> +}
> +
> +/* 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 greater performance drop but for a shorter
> +   duration.
> +*/
> +void *migration_throttle_down(void *opaque)
> +{
> +    throttling = true;
> +    while (throttling_needed()) {
> +        CPUArchState *penv = first_cpu;
> +        while (penv) {
> +            qemu_mutex_lock_iothread();
> +            run_on_cpu(ENV_GET_CPU(penv), mig_kick_cpu, NULL);
> +            qemu_mutex_unlock_iothread();
> +            penv = penv->next_cpu;
> +        }
> +        g_usleep(25*1000);
> +    }
> +    throttling = false;
> +    return NULL;
> +}
> diff --git a/migration.c b/migration.c
> index 3eb0fad..834156e 100644
> --- a/migration.c
> +++ b/migration.c
> @@ -24,6 +24,7 @@
>  #include "qemu/thread.h"
>  #include "qmp-commands.h"
>  #include "trace.h"
> +#include "sysemu/cpus.h"
>  
>  //#define DEBUG_MIGRATION
>  
> @@ -474,6 +475,15 @@ void qmp_migrate_set_downtime(double value, Error **errp)
>      max_downtime = (uint64_t)value;
>  }
>  
> +bool migrate_auto_converge(void)
> +{
> +    MigrationState *s;
> +
> +    s = migrate_get_current();
> +
> +    return s->enabled_capabilities[MIGRATION_CAPABILITY_AUTO_CONVERGE];
> +}
> +
>  int migrate_use_xbzrle(void)
>  {
>      MigrationState *s;
> @@ -503,6 +513,7 @@ static void *migration_thread(void *opaque)
>      int64_t max_size = 0;
>      int64_t start_time = initial_time;
>      bool old_vm_running = false;
> +    QemuThread thread;
>  
>      DPRINTF("beginning savevm\n");
>      qemu_savevm_state_begin(s->file, &s->params);
> @@ -517,6 +528,10 @@ static void *migration_thread(void *opaque)
>              DPRINTF("pending size %lu max %lu\n", pending_size, max_size);
>              if (pending_size && pending_size >= max_size) {
>                  qemu_savevm_state_iterate(s->file);
> +                if (throttling_needed() && !throttling_now()) {
> +                    qemu_thread_create(&thread, migration_throttle_down,
> +                               NULL, QEMU_THREAD_DETACHED);
> +                }
>              } else {
>                  DPRINTF("done iterating\n");
>                  qemu_mutex_lock_iothread();
> diff --git a/qapi-schema.json b/qapi-schema.json
> index 5b0fb3b..b662e33 100644
> --- a/qapi-schema.json
> +++ b/qapi-schema.json
> @@ -599,10 +599,14 @@
>  #          This feature allows us to minimize migration traffic for certain 
> work
>  #          loads, by sending compressed difference of the pages
>  #
> +# @auto-converge: Controls whether or not the we want the migration to
> +#          automaticially detect and force convergence by slowing
> +#          down the guest. Disabled by default.
> +#
>  # Since: 1.2
>  ##
>  { 'enum': 'MigrationCapability',
> -  'data': ['xbzrle'] }
> +  'data': ['xbzrle', 'auto-converge'] }
>  
>  ##
>  # @MigrationCapabilityStatus
>