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

[Igor Mammedov]

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.

> Regards,
> Daniel