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

[Jitendra Kolhe]

On 1/27/2017 6:23 PM, Juan Quintela wrote:
> Jitendra Kolhe <address@hidden> wrote:
>> Using "-mem-prealloc" option for a very large guest leads to huge 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. Given the problem is more
>> prominent for large guests, the patch limits the changes to the guests
>> of at-least 64GB of memory size. 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
>                     ^^^^^^^^^
> 
> Impressive, not everyday one get an speedup of 20 O:-)
> 
> 
>> +static void *do_touch_pages(void *arg)
>> +{
>> +    PageRange *range = (PageRange *)arg;
>> +    char *start_addr = range->addr;
>> +    uint64_t numpages = range->numpages;
>> +    uint64_t hpagesize = range->hpagesize;
>> +    uint64_t i = 0;
>> +
>> +    for (i = 0; i < numpages; i++) {
>> +        memset(start_addr + (hpagesize * i), 0, 1);
> 
> I would use the range->addr and similar here directly, but it is just a
> question of taste.
> 

Thanks for your response, 
will update my next patch.

>> -        /* MAP_POPULATE silently ignores failures */
>> -        for (i = 0; i < numpages; i++) {
>> -            memset(area + (hpagesize * i), 0, 1);
>> +        /* touch pages simultaneously for memory >= 64G */
>> +        if (memory < (1ULL << 36)) {
> 
> 64GB guest already took quite a bit of time, I think I would put it
> always as min(num_vcpus, 16).  So, we always execute the multiple theard
> codepath?
> 

It sounds good idea to have a heuristic on vcpu count. I will add in my 
next version. But shouldn't we also restrict based on guest RAM size too, 
to avoid overhead spawning multiple threads for thin guests?

Thanks,
- Jitendra

> But very nice, thanks.
> 
> Later, Juan.
>