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[[!tag open_issue_gnumach open_issue_hurd]]
IRC, freenode, #hurd, 2011-02-16
<braunr> exceptfor the kernel, everything in an address space is
represented with a VM object
<braunr> those objects can represent anonymous memory (from malloc() or
because of a copy-on-write)
<braunr> or files
<braunr> on classic Unix systems, these are files
<braunr> on the Hurd, these are memory objects, backed by external pagers
(like ext2fs)
<braunr> so when you read a file
<braunr> the kernel maps it from ext2fs in your address space
<braunr> and when you access the memory, a fault occurs
<braunr> the kernel determines it's a region backed by ext2fs
<braunr> so it asks ext2fs to provide the data
<braunr> when the fault is resolved, your process goes on
<etenil> does the faul occur because Mach doesn't know how to access the
memory?
<braunr> it occurs because Mach intentionnaly didn't back the region with
physical memory
<braunr> the MMU is programmed not to know what is present in the memory
region
<braunr> or because it's read only
<braunr> (which is the case for COW faults)
<etenil> so that means this bit of memory is a buffer that ext2fs loads the
file into and then it is remapped to the application that asked for it
<braunr> more or less, yes
<braunr> ideally, it's directly written into the right pages
<braunr> there is no intermediate buffer
<etenil> I see
<etenil> and as you told me before, currently the page faults are handled
one at a time
<etenil> which wastes a lot of time
<braunr> a certain amount of time
<etenil> enough to bother the user :)
<etenil> I've seen pages have a fixed size
<braunr> yes
<braunr> use the PAGE_SIZE macro
<etenil> and when allocating memory, the size that's asked for is rounded
up to the page size
<etenil> so if I have this correctly, it means that a file ext2fs provides
could be split into a lot of pages
<braunr> yes
<braunr> once in memory, it is managed by the page cache
<braunr> so that pages more actively used are kept longer than others
<braunr> in order to minimize I/O
<etenil> ok
<braunr> so a better page cache code would also improve overall performance
<braunr> and more RAM would help a lot, since we are strongly limited by
the 768 MiB limit
<braunr> which reduces the page cache size a lot
<etenil> but the problem is that reading a whole file in means trigerring
many page faults just for one file
<braunr> if you want to stick to the page clustering thing, yes
<braunr> you want less page faults, so that there are less IPC between the
kernel and the pager
<etenil> so either I make pages bigger
<etenil> or I modify Mach so it can check up on a range of pages for faults
before actually processing
<braunr> you *don't* change the page size
<etenil> ah
<etenil> that's hardware isn't it?
<braunr> in Mach, yes
<etenil> ok
<braunr> and usually, you want the page size to be the CPU page size
<etenil> I see
<braunr> current CPU can support multiple page sizes, but it becomes quite
hard to correctly handle
<braunr> and bigger page sizes mean more fragmentation, so it only suits
machines with large amounts of RAM, which isn't the case for us
<etenil> ok
<etenil> so I'll try the second approach then
<braunr> that's what i'd recommand
<braunr> recommend*
<etenil> ok
---
IRC, freenode, #hurd, 2011-02-16
<antrik> etenil: OSF Mach does have clustered paging BTW; so that's one
place to start looking...
<antrik> (KAM ported the OSF code to gnumach IIRC)
<antrik> there is also an existing patch for clustered paging in libpager,
which needs some adaptation
<antrik> the biggest part of the task is probably modifying the Hurd
servers to use the new interface
<antrik> but as I said, KAM's code should be available through google, and
can serve as a starting point
<http://lists.gnu.org/archive/html/bug-hurd/2010-06/msg00023.html>
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