Date: Sat, 28 Mar 2009 19:01:16 -0700 From: Suleiman Souhlal <ssouhlal@FreeBSD.org> To: Scott Long <scottl@samsco.org> Cc: Sergey Babkin <babkin@verizon.net>, freebsd-hackers@FreeBSD.org, attilio@FreeBSD.org, phk@phk.freebsd.dk, freebsd-current@FreeBSD.org, Robert Watson <rwatson@FreeBSD.org>, prashant.vaibhav@gmail.com Subject: Re: Improving the kernel/i386 timecounter performance (GSoC proposal) Message-ID: <04EDFED9-24B4-404C-96F7-2C96FBC300B4@FreeBSD.org> In-Reply-To: <49CD1B3D.3030103@samsco.org> References: <11609492.9579.1238167614335.JavaMail.root@vms070.mailsrvcs.net> <49CD0405.1060704@samsco.org> <alpine.BSF.2.00.0903271821060.60642@fledge.watson.org> <49CD1B3D.3030103@samsco.org>
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On Mar 27, 2009, at 11:30 AM, Scott Long wrote: > Robert Watson wrote: >> On Fri, 27 Mar 2009, Scott Long wrote: >>> I've been talking about this for years. All I need is help with >>> the VM magic to create the page on fork. I also want two pages, >>> one global for gettimeofday (and any other global data we can >>> think of) and one per-process for static data like getpid/getgid. >> FWIW, there are some variations in schemes across OS's -- one >> extreme is the Linux approach, which actually exports a mini >> shared library in ELF format on the shared page, providing >> implementations of various services (such as entering system >> calls), time stuff, etc. Less extreme are the shared pages >> offered on Mac OS X, etc. > > Yes, but I'd like to start somewhere, and considering that it's been > impossible in _5_ years to get the 30 minutes of Peter or JeffR or JHB > time to get the basic VM magic done, I'm keeping my expectations as > modest as possible. > You can find a proof-of-concept implementation for amd64 of a global page mapped in every process at http://people.freebsd.org/~ssouhlal/ testing/syspage-20090328.diff . It exports ticks to userland at VM_MIN_KERNEL_ADDRESS (0xfffffffe40000000). In order for this to work on architectures without a direct map, the page will need to be mapped a second time as read/write (you might want to have a vm_offset_t pmap_map_syspage(vm_page_t m) function that does the right thing for each architecture). Unfortunately, this trick probably won't work for per-process pages without more work, because we wouldn't be able to just insert the page in kernel_map. -- Suleiman
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