On 2/13/2017 5:34 PM, Igor Mammedov wrote:
> On Mon, 13 Feb 2017 11:23:17 +0000
> "Daniel P. Berrange" <berra...@redhat.com> 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 <jitendra.ko...@hpe.com> 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 <jitendra.ko...@hpe.com>
>>>> ---
>>>> 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
>
