On Mon, Jul 09, 2012 at 02:11:00AM -0400, Tom Lane wrote: > y...@mwd.biglobe.ne.jp (YAMAMOTO Takashi) writes: > >> Also, I was under the impression that recent Linux kernels use hugepages > >> automatically if they can, so I wonder exactly what Andres was testing > >> on ... > > > if you mean the "trasparent hugepage" feature, iirc it doesn't affect > > MAP_SHARED mappings like this. > > Oh! That would explain some things. It seems like a pretty nasty > restriction though ... do you know why they did that?
It doesn't say explicitly in the documentation (found at http://lwn.net/Articles/423592/ aka transhuge.txt) but reading between the lines I'm guessing it's due to the fact that huge pages must be aligned to 2 or 4MB and when dealing with a shared mapping you probably need to require it to be aligned is all address spaces. However, it seems it should work for SysV shared memory, see: http://lwn.net/Articles/375096/ . The same page suggests shared mappings should work fine. However, this page refers to the non-transparent feature. If you think about it, it must work since huge pages are inherited through fork(). Have a nice day, -- Martijn van Oosterhout <klep...@svana.org> http://svana.org/kleptog/ > He who writes carelessly confesses thereby at the very outset that he does > not attach much importance to his own thoughts. -- Arthur Schopenhauer
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