Date: Fri, 28 Oct 2011 00:38:04 -0500 From: Alan Cox <alc@rice.edu> To: Svatopluk Kraus <onwahe@gmail.com> Cc: alc@freebsd.org, Wojciech Puchar <wojtek@wojtek.tensor.gdynia.pl>, Kostik Belousov <kostikbel@gmail.com>, hackers@freebsd.org, Grzegorz Kulewski <grzegorz@kulewski.pl> Subject: Re: mmap performance and memory use Message-ID: <4EAA3FBC.3090907@rice.edu> In-Reply-To: <CAFHCsPXb4Z0NP49OFRttwaAeagE_JV0-QYTTTUzimV7MVfxXFg@mail.gmail.com> References: <alpine.BSF.2.00.1110061637270.15552@wojtek.tensor.gdynia.pl> <20111006160159.GQ1511@deviant.kiev.zoral.com.ua> <CAJUyCcMKZ1E5LoJ0BFZju_OH_jjDeqgvBMf8WZ%2BwDvJRFsbH6Q@mail.gmail.com> <alpine.BSF.2.00.1110071916190.8664@wojtek.tensor.gdynia.pl> <4E8FF4B8.7010300@rice.edu> <alpine.BSF.2.00.1110102325220.67653@wojtek.tensor.gdynia.pl> <4EA747B5.9040304@rice.edu> <CAFHCsPXb4Z0NP49OFRttwaAeagE_JV0-QYTTTUzimV7MVfxXFg@mail.gmail.com>
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On 10/26/2011 06:23, Svatopluk Kraus wrote: > Hi, > > well, I'm working on new port (arm11 mpcore) and pmap_enter_object() > is what I'm debugging rigth now. And I did not find any way in > userland how to force kernel to call pmap_enter_object() which makes > SUPERPAGE mapping without promotion. I tried to call mmap() with > MAP_PREFAULT_READ without success. I tried to call madvise() with > MADV_WILLNEED without success too. > mmap() should call pmap_enter_object() if MAP_PREFAULT_READ was specified. I'm surprised to hear that it's not happening for you. > To make SUPERPAGE mapping, it's obvious that all physical pages under > SUPERPAGE must be allocated in vm_object. And SUPERPAGE mapping must > be done before first access to them, otherwise a promotion is on the > way. MAP_PREFAULT_READ does nothing with it. If madvice() is used, > vm_object_madvise() is called but only cached pages are allocated in > advance. Of coarse, an allocation of all physical memory behind > virtual address space in advance is not preferred in most situations. > > For example, I want to do some computation on 4M memory space (I know > that each byte will be accessed) and want to utilize SUPERPAGE mapping > without promotion, so save 4K page table (i386 machine). However, > malloc() leads to promotion, mmap() with MAP_PREFAULT_READ doesn't do > nothing so SUPERPAGE mapping is promoted, and madvice() with > MADV_WILLNEED calls vm_object_madvise() but because the pages are not > cached (how can be on anonymous memory), it is not work without > promotion too. > > So, SUPERPAGE mapping without promotions is fine, but it can be done > only if physical memory being mapped is already allocated. Is it > really possible to force that in userland? > To force the allocation of the physical memory? Right now, the only way is for your program to touch the pages. > Moreover, the SUPERPAGE mapping is made readonly firstly. So, even if > I have SUPERPAGE mapping without promotion, the mapping is demoted > after first write, and promoted again after all underlying pages are > accessed by write. There is 4K page table saving no longer. > Yes, that is all true. It is possible to change things so that the page table pages are reclaimed after a time, and not kept around indefinitely. However, this not high on my personal priority list. Before that, it is more likely that I will add an option to avoid the demotion on write, if we don't have to copy the entire superpage to do so. Alan
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