Re: [Qemu-devel] [PATCH] Add option to mlock guest and qemu memory

[Anthony Liguori]

Jan Kiszka <address@hidden> writes:

> On 2012-09-28 01:21, Satoru Moriya wrote:
>> This is a first time for me to post a patch to qemu-devel.
>> If there is something missing/wrong, please let me know.
>> 
>> We have some plans to migrate old enterprise systems which require
>> low latency (msec order) to kvm virtualized environment. Usually,
>> we uses mlock to preallocate and pin down process memory in order
>> to avoid page allocation in latency critical path. On the other
>> hand, in kvm environment, mlocking in guests is not effective
>> because it can't avoid page reclaim in host. Actually, to avoid
>> guest memory reclaim, qemu has "mem-path" option that is actually
>> for using hugepage. But a memory region of qemu is not allocated
>> on hugepage, so it may be reclaimed. That may cause a latency
>> problem.
>> 
>> To avoid guest and qemu memory reclaim, this patch introduces
>> a new "mlock" option. With this option, we can preallocate and
>> pin down guest and qemu memory before booting guest OS.
>
> I guess this reduces the likeliness of multi-millisecond latencies for
> you but not eliminate them. Of course, mlockall is part of our local
> changes for real-time QEMU/KVM, but it is just one of the many pieces
> required. I'm wondering how the situation is on your side.
>
> I think mlockall should once be enabled automatically as soon as you ask
> for real-time support for QEMU guests. How that should be controlled is
> another question. I'm currently carrying a top-level switch "-rt
> maxprio=x[,policy=y]" here, likely not the final solution. I'm not
> really convinced we need to control memory locking separately. And as we
> are very reluctant to add new top-level switches, this is even more
> important.

I think you're right here although I'd suggest not abbreviating.

Regards,

Anthony Liguori

>
>> 
>> Tested on Linux, x86_64 (fedora 17).
>> 
>> Signed-off-by: Satoru Moriya <address@hidden>
>> ---
>>  cpu-all.h       | 1 +
>>  exec.c          | 3 +++
>>  qemu-options.hx | 8 ++++++++
>>  vl.c            | 4 ++++
>>  4 files changed, 16 insertions(+)
>> 
>> diff --git a/cpu-all.h b/cpu-all.h
>> index 74d3681..e12e5d5 100644
>> --- a/cpu-all.h
>> +++ b/cpu-all.h
>> @@ -503,6 +503,7 @@ extern RAMList ram_list;
>>  
>>  extern const char *mem_path;
>>  extern int mem_prealloc;
>> +extern int mem_lock;
>>  
>>  /* Flags stored in the low bits of the TLB virtual address.  These are
>>     defined so that fast path ram access is all zeros.  */
>> diff --git a/exec.c b/exec.c
>> index bb6aa4a..de13bc9 100644
>> --- a/exec.c
>> +++ b/exec.c
>> @@ -2572,6 +2572,9 @@ ram_addr_t qemu_ram_alloc_from_ptr(ram_addr_t size, 
>> void *host,
>>              }
>>              memory_try_enable_merging(new_block->host, size);
>>          }
>> +        if (mem_lock && mlockall(MCL_CURRENT | MCL_FUTURE)) {
>> +            perror("mlockall");
>> +        }
>
> This belongs to the OS abstraction layer (it's POSIX). And you only need
> to call it once per process lifetime.
>
>>      }
>>      new_block->length = size;
>>  
>> diff --git a/qemu-options.hx b/qemu-options.hx
>> index 7d97f96..9d82f15 100644
>> --- a/qemu-options.hx
>> +++ b/qemu-options.hx
>> @@ -427,6 +427,14 @@ Preallocate memory when using -mem-path.
>>  ETEXI
>>  #endif
>>  
>> +DEF("mlock", 0, QEMU_OPTION_mlock,
>> +    "-mlock          mlock guest and qemu memory\n",
>> +    QEMU_ARCH_ALL)
>> +STEXI
>> address@hidden -mlock
>> +mlock guest and qemu memory.
>> +ETEXI
>> +
>>  DEF("k", HAS_ARG, QEMU_OPTION_k,
>>      "-k language     use keyboard layout (for example 'fr' for French)\n",
>>      QEMU_ARCH_ALL)
>> diff --git a/vl.c b/vl.c
>> index 8d305ca..c902084 100644
>> --- a/vl.c
>> +++ b/vl.c
>> @@ -187,6 +187,7 @@ const char *mem_path = NULL;
>>  #ifdef MAP_POPULATE
>>  int mem_prealloc = 0; /* force preallocation of physical target memory */
>>  #endif
>> +int mem_lock;
>>  int nb_nics;
>>  NICInfo nd_table[MAX_NICS];
>>  int autostart;
>> @@ -2770,6 +2771,9 @@ int main(int argc, char **argv, char **envp)
>>                  mem_prealloc = 1;
>>                  break;
>>  #endif
>> +            case QEMU_OPTION_mlock:
>> +                mem_lock = 1;
>> +                break;
>>              case QEMU_OPTION_d:
>>                  log_mask = optarg;
>>                  break;
>> 
>
> Jan
>
> -- 
> Siemens AG, Corporate Technology, CT RTC ITP SDP-DE
> Corporate Competence Center Embedded Linux