Re: [Qemu-devel] [PULL 03/18] mem-prealloc: reduce large guest start-up and migration time.

[Jitendra Kolhe]

On 3/18/2017 7:28 PM, Peter Maydell wrote:
> On 14 March 2017 at 16:18, Paolo Bonzini <address@hidden> wrote:
>> From: Jitendra Kolhe <address@hidden>
>>
>> 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.
> 
>> --- a/util/oslib-posix.c
>> +++ b/util/oslib-posix.c
>> @@ -55,6 +55,21 @@
>>  #include "qemu/error-report.h"
>>  #endif
>>
>> +#define MAX_MEM_PREALLOC_THREAD_COUNT (MIN(sysconf(_SC_NPROCESSORS_ONLN), 
>> 16))
> 
> sysconf() can fail, in which case it will return -1...
> 
>> +static bool touch_all_pages(char *area, size_t hpagesize, size_t numpages,
>> +                            int smp_cpus)
>> +{
>> +    uint64_t numpages_per_thread, size_per_thread;
>> +    char *addr = area;
>> +    int i = 0;
>> +
>> +    memset_thread_failed = false;
>> +    memset_num_threads = MIN(smp_cpus, MAX_MEM_PREALLOC_THREAD_COUNT);
> 
> ...and memset_num_threads (being signed) will also end up -1 here...
> 
>> +    memset_thread = g_new0(MemsetThread, memset_num_threads);
> 
> ...which we then pass to g_new0(), which will not have good
> results.
> 
> (Spotted by Coverity, CID 1372465.)
> 
> Hiding a sysconf() behind a macro that looks like it's going
> to be a constant integer is a bit confusing, incidentally,
> and we only use it in one place. I'd just have a memset_num_threads()
> function which calls sysconf and determines the right number from
> that and smp_cpus and 16, and handles sysconf failing gracefully.
> 

Thanks, posted a patch with the changes.

- Jitendra

> thanks
> -- PMM
>