mmap performance and memory use

Alan Cox alc at
Tue Oct 11 16:12:46 UTC 2011

On 10/10/2011 16:28, Wojciech Puchar wrote:
>> Notice that vm.pmap.pde.promotions increased by 31.  This means that 
>> 31 superpage mappings were created by promotion from small page 
>> mappings.
> thank you. i looked at .mappings as it seemed logical for me that is 
> shows total.
>> In contrast, vm.pmap.pde.mappings counts superpage mappings that are 
>> created directly and not by promotion from small page mappings.  For 
>> example, if a large executable, such as gcc, is resident in memory, 
>> the text segment will be pre-mapped using superpage mappings, 
>> avoiding soft fault and promotion overhead.  Similarly, mmap(..., 
>> MAP_PREFAULT_READ) on a large, memory resident file may pre-map the 
>> file using superpage mappings.
> your options are not described in mmap manpage nor madvise 
> when can i find the up to date manpage or description?

It is documented in mmap(2) on HEAD and 9.x:

                        Immediately update the calling process's 
                        virtual address translation structures, such as its
                        page table, so that every memory resident page 
                        the region is mapped for read access.  
Ordinarily these
                        structures are updated lazily.  The effect of this
                        option is to eliminate any soft faults that 
would oth-
                        erwise occur on the initial read accesses to the
                        region.  Although this option does not preclude prot
                        from including PROT_WRITE, it does not eliminate 
                        faults on the initial write accesses to the region.

I don't believe that this feature was merged into to 8.x.  However, 
there is no technical reason that it can't be merged.

> is it possible to force VM subsystem to operate on superpages when 
> possible - i mean swapping in 2MB chunks?

Currently, no.  For some applications, like the Sun/Oracle JVM, that 
have code to explicitly manage large pages, there could be some benefit 
in the form of reduced overhead.  So, it's on my "to do" list, but no 
where near the top of that list.


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