c++ - Relationship between dwPageSize and dwAllocationGranularity -


i’m reading google’s tcmalloc source code (the windows porting). 

int getpagesize()  {     static int pagesize = 0;     if (pagesize == 0)      {       system_info system_info;       getsysteminfo(&system_info);       pagesize = std::max(system_info.dwpagesize, system_info.dwallocationgranularity);     }     return pagesize;  }

as can se in code snippet above pagesize(that unit of allocation) calculated max between dwpagesize , dwallocationgranularity. mean know kind of relationship between these 2 values: necessary calculate value in way here upside explicated? , there situations in dwpagesize greater dwallocationgranularity?

disclaimer: answer not based on documentation on interpretation of these constants.

i assume page size correctly reported. assume allocation granularity refers granularity of os memory allocation interface.

there these 2 cases consider:

  • the allocation granularity greater page size. allocating memory block of size of page lead larger allocation of resources, should prevented.

  • the allocation granularity less page size. allocating memory block of size of allocation granularity still lead whole page being allocated/mapped, should prevented.

basically both cases cause os allocate more memory requested. using maximum can avoided, such (user space) allocation code can (relatively) actual memory usage.


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