Date: Fri, 7 Oct 2011 12:13:31 -0500 From: Alan Cox <alan.l.cox@gmail.com> To: Kostik Belousov <kostikbel@gmail.com> Cc: Wojciech Puchar <wojtek@wojtek.tensor.gdynia.pl>, hackers@freebsd.org, Grzegorz Kulewski <grzegorz@kulewski.pl> Subject: Re: mmap performance and memory use Message-ID: <CAJUyCcMKZ1E5LoJ0BFZju_OH_jjDeqgvBMf8WZ%2BwDvJRFsbH6Q@mail.gmail.com> In-Reply-To: <20111006160159.GQ1511@deviant.kiev.zoral.com.ua> References: <alpine.BSF.2.00.1110061637270.15552@wojtek.tensor.gdynia.pl> <20111006160159.GQ1511@deviant.kiev.zoral.com.ua>
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On Thu, Oct 6, 2011 at 11:01 AM, Kostik Belousov <kostikbel@gmail.com>wrote: > On Thu, Oct 06, 2011 at 04:41:45PM +0200, Wojciech Puchar wrote: > > i have few questions. > > > > 1) suppose i map 1TB of address space as anonymous and touch just one > > page. how much memory is used to manage this? > I am not sure how deep the enumeration you want to know, but the first > approximation will be: > one struct vm_map_entry > one struct vm_object > one pv_entry > > Page table structures need four pages for directories and page table > proper. > > > > 2) suppose we have 1TB file on disk without holes and 100000 processes > > mmaps this file to it's address space. are just pages shared or can > > pagetables be shared too? how much memory is used to manage such > > situation? > Only pages are shared. Pagetables are not. > > 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 mapping. Alan
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