mmap performance and memory use
ivoras at freebsd.org
Tue Oct 11 10:05:55 UTC 2011
On 07/10/2011 19:13, Alan Cox wrote:
> On Thu, Oct 6, 2011 at 11:01 AM, Kostik Belousov <kostikbel at gmail.com>wrote:
>> For one thing, this indeed causes more memory use for the OS. This is
>> somewhat mitigated by automatic use of superpages. Superpage promotion
>> still keeps the 4KB page table around, so most savings from the
>> superpages are due to more efficient use of TLB.
> You are correct about the page table page. However, a superpage mapping
> consumes a single PV entry, in place of 512 or 1024 PV entries. This winds
> up saving about three physical pages worth of memory for every superpage
But wouldn't the "conservative" superpages policy make it difficult in
the OPs case to ever get promotions to superpages if he's touching pages
> Similarly, mmap(..., MAP_PREFAULT_READ) on a large, memory resident file may pre-map the file using superpage mappings.
grep doesn't find this symbol in the sys src tree in 8-STABLE - nor it
seems in /usr/include.
But anyway, is there a mechanism which gives more guarantees than "may"
(i.e. which forces this) - or if not, how hard would it be to add one?
Some Linux-based "enterprise" software (including Java) use
Linux-specific calls to allocate large pages directly.
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