mmap performance and memory use

Alan Cox alc at
Wed Oct 12 16:24:45 UTC 2011

On 10/11/2011 12:36, Mark Tinguely wrote:
> On 10/11/2011 11:12 AM, Alan Cox wrote:
>> On 10/10/2011 16:28, Wojciech Puchar wrote:
>>> 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.
>> Alan
> Am I correct in remembering that super-pages have to be aligned on the 
> super-page boundary and be contiguous?

Yes.  However, if you allocate (or mmap(2)) a large range of virtual 
memory, e.g., 10 MB, and the start of that range is not aligned on a 
superpage boundary, the virtual memory system can still promote the four 
2 MB sized superpages in the middle of that range.

> If so, in the mmap(), he may want to include the 'MAP_FIXED' flag with 
> an address that is on a super-page boundary. Right now, the 
> "VMFS_ALIGNED_SPACE" that does the VA super-page alignment is only 
> used for device pagers.

Yes.  More precisely, the second, third, etc. mmap(2) should duplicate 
the alignment of the first mmap(2).  In fact, this is what 
VMFS_ALIGNED_SPACE does.  It looks at the alignment of the pages already 
allocated to the file (or vm object) and attempts to duplicate that 

Sooner or later, I will probably make VMFS_ALIGNED_SPACE the default for 
file types other than devices.

> Similarly, if the allocated physical pages for the object are not 
> contiguous, then MAP_PREFAULT_READ will not result in a super-page 
> promotion.

As described in my earlier e-mail on this topic, in this case, I call 
these superpage mappings and not superpage promotions, because the 
virtual system creates a large page mapping, e.g., a 2 MB page table 
entry, from the start.  It does not create small page mappings and then 
promote them to a large page mapping.


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