Re: [Qemu-devel] [PATCH v2] mem-prealloc: reduce large guest start-up and migration time.

[Jitendra Kolhe]

On 2/13/2017 5:34 PM, Igor Mammedov wrote:
> On Mon, 13 Feb 2017 11:23:17 +0000
> "Daniel P. Berrange" <address@hidden> wrote:
> 
>> On Mon, Feb 13, 2017 at 11:45:46AM +0100, Igor Mammedov wrote:
>>> On Mon, 13 Feb 2017 14:30:56 +0530
>>> Jitendra Kolhe <address@hidden> wrote:
>>>   
>>>> Using "-mem-prealloc" option for a large guest leads to higher guest
>>>> start-up and migration time. This is because with "-mem-prealloc" option
>>>> qemu tries to map every guest page (create address translations), and
>>>> make sure the pages are available during runtime. virsh/libvirt by
>>>> default, seems to use "-mem-prealloc" option in case the guest is
>>>> configured to use huge pages. The patch tries to map all guest pages
>>>> simultaneously by spawning multiple threads. Currently limiting the
>>>> change to QEMU library functions on POSIX compliant host only, as we are
>>>> not sure if the problem exists on win32. Below are some stats with
>>>> "-mem-prealloc" option for guest configured to use huge pages.
>>>>
>>>> ------------------------------------------------------------------------
>>>> Idle Guest      | Start-up time | Migration time
>>>> ------------------------------------------------------------------------
>>>> Guest stats with 2M HugePage usage - single threaded (existing code)
>>>> ------------------------------------------------------------------------
>>>> 64 Core - 4TB   | 54m11.796s    | 75m43.843s
>>>> 64 Core - 1TB   | 8m56.576s     | 14m29.049s
>>>> 64 Core - 256GB | 2m11.245s     | 3m26.598s
>>>> ------------------------------------------------------------------------
>>>> Guest stats with 2M HugePage usage - map guest pages using 8 threads
>>>> ------------------------------------------------------------------------
>>>> 64 Core - 4TB   | 5m1.027s      | 34m10.565s
>>>> 64 Core - 1TB   | 1m10.366s     | 8m28.188s
>>>> 64 Core - 256GB | 0m19.040s     | 2m10.148s
>>>> -----------------------------------------------------------------------
>>>> Guest stats with 2M HugePage usage - map guest pages using 16 threads
>>>> -----------------------------------------------------------------------
>>>> 64 Core - 4TB   | 1m58.970s     | 31m43.400s
>>>> 64 Core - 1TB   | 0m39.885s     | 7m55.289s
>>>> 64 Core - 256GB | 0m11.960s     | 2m0.135s
>>>> -----------------------------------------------------------------------
>>>>
>>>> Changed in v2:
>>>>  - modify number of memset threads spawned to min(smp_cpus, 16).
>>>>  - removed 64GB memory restriction for spawning memset threads.
>>>>
>>>> Signed-off-by: Jitendra Kolhe <address@hidden>
>>>> ---
>>>>  backends/hostmem.c   |  4 ++--
>>>>  exec.c               |  2 +-
>>>>  include/qemu/osdep.h |  3 ++-
>>>>  util/oslib-posix.c   | 68 
>>>> +++++++++++++++++++++++++++++++++++++++++++++++-----
>>>>  util/oslib-win32.c   |  3 ++-
>>>>  5 files changed, 69 insertions(+), 11 deletions(-)
>>>>
>>>> diff --git a/backends/hostmem.c b/backends/hostmem.c
>>>> index 7f5de70..162c218 100644
>>>> --- a/backends/hostmem.c
>>>> +++ b/backends/hostmem.c
>>>> @@ -224,7 +224,7 @@ static void host_memory_backend_set_prealloc(Object 
>>>> *obj, bool value,
>>>>          void *ptr = memory_region_get_ram_ptr(&backend->mr);
>>>>          uint64_t sz = memory_region_size(&backend->mr);
>>>>  
>>>> -        os_mem_prealloc(fd, ptr, sz, &local_err);
>>>> +        os_mem_prealloc(fd, ptr, sz, smp_cpus, &local_err);
>>>>          if (local_err) {
>>>>              error_propagate(errp, local_err);
>>>>              return;
>>>> @@ -328,7 +328,7 @@ host_memory_backend_memory_complete(UserCreatable *uc, 
>>>> Error **errp)
>>>>           */
>>>>          if (backend->prealloc) {
>>>>              os_mem_prealloc(memory_region_get_fd(&backend->mr), ptr, sz,
>>>> -                            &local_err);
>>>> +                            smp_cpus, &local_err);
>>>>              if (local_err) {
>>>>                  goto out;
>>>>              }
>>>> diff --git a/exec.c b/exec.c
>>>> index 8b9ed73..53afcd2 100644
>>>> --- a/exec.c
>>>> +++ b/exec.c
>>>> @@ -1379,7 +1379,7 @@ static void *file_ram_alloc(RAMBlock *block,
>>>>      }
>>>>  
>>>>      if (mem_prealloc) {
>>>> -        os_mem_prealloc(fd, area, memory, errp);
>>>> +        os_mem_prealloc(fd, area, memory, smp_cpus, errp);
>>>>          if (errp && *errp) {
>>>>              goto error;
>>>>          }
>>>> diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
>>>> index 56c9e22..fb1d22b 100644
>>>> --- a/include/qemu/osdep.h
>>>> +++ b/include/qemu/osdep.h
>>>> @@ -401,7 +401,8 @@ unsigned long qemu_getauxval(unsigned long type);
>>>>  
>>>>  void qemu_set_tty_echo(int fd, bool echo);
>>>>  
>>>> -void os_mem_prealloc(int fd, char *area, size_t sz, Error **errp);
>>>> +void os_mem_prealloc(int fd, char *area, size_t sz, int smp_cpus,
>>>> +                     Error **errp);
>>>>  
>>>>  int qemu_read_password(char *buf, int buf_size);
>>>>  
>>>> diff --git a/util/oslib-posix.c b/util/oslib-posix.c
>>>> index f631464..17da029 100644
>>>> --- a/util/oslib-posix.c
>>>> +++ b/util/oslib-posix.c
>>>> @@ -55,6 +55,16 @@
>>>>  #include "qemu/error-report.h"
>>>>  #endif
>>>>  
>>>> +#define MAX_MEM_PREALLOC_THREAD_COUNT 16  
>>> running with -smp 16 or bigger on host with less than 16 cpus
>>> it would be not quite optimal.
>>> Why not to change MAX_MEM_PREALLOC_THREAD_COUNT constant to
>>> something like sysconf(_SC_NPROCESSORS_ONLN)  
>>
>> The point is to not consume more host resources than would otherwise
>> be consumed by running the guest CPUs. ie, if running a KVM guest
>> with -smp 4 on a 16 CPU host,  QEMU should not to consume more than
>> 4 pCPUs worth of resource on the host.  Using sysconf would cause
>> the consume to consume all host resources, likely harming other
>> guests workloads.
>>
>> If the person launching QEMU gives a -smp value that's larger than
>> the host CPUs count, then they've already accepted that they're
>> asking QEMU todo more than the host is really capable of. IOW, I
>> don't think we need to special case memsetting for that, since
>> VCPU execution itself is already going to overcommit the host.
> Doing over commit at preallocate time doesn't make much sense,
> if MAX_MEM_PREALLOC_THREAD_COUNT is replaced with 
> sysconf(_SC_NPROCESSORS_ONLN)
> then QEMU will end up with MIN(-smp, sysconf(_SC_NPROCESSORS_ONLN))
> which will put cap on upper value and avoid useless over commit at
> preallocate time.
> 

I agree, we should consider case where we run with -smp >= 16 which 
is overcommited on host with < 16 cpus. At the same time we should 
also be sure that we don't end up spawning to many memset threads. 
For e.g. I have been running fat guests with -smp > 64 on hosts 
with 384 cpus.

Thanks,
- Jitendra

>> Regards,
>> Daniel
>