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

[Paolo Bonzini]

On 27/01/2017 13:53, Juan Quintela wrote:
>> +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.
> 
>> -        /* 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?

I too would like some kind of heuristic to choose the number of threads.
 Juan's suggested usage of the VCPUs (smp_cpus) is a good one.

Paolo