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[[!meta copyright="Copyright © 2011 Free Software Foundation, Inc."]]
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[[!tag open_issue_gnumach open_issue_hurd]]
[[community/gsoc/project_ideas/disk_io_performance]].
[[!toc]]
# 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>
# IRC, freenode, #hurd, 2011-07-22
<braunr> but concerning clustered pagins/outs, i'm not sure it's a mach
interface limitation
<braunr> the external memory pager interface does allow multiple pages to
be transfered
<braunr> isn't it an internal Mach VM problem ?
<braunr> isn't it simply the page fault handler ?
<antrik> braunr: are you sure? I was under the impression that changing the
pager interface was among the requirements...
<antrik> hm... I wonder whether for pageins, it could actually be handled
in the pages instead of Mach... though this wouldn't work for pageouts,
so probably not very helpful
<antrik> err... in the pagers
<braunr> antrik: i'm almost sure
<braunr> but i've be proven wrong many times, so ..
<braunr> there are two main facts that lead me to think this
<braunr> 1/
http://www.gnu.org/software/hurd/gnumach-doc/Memory-Objects-and-Data.html#Memory-Objects-and-Data
says lengths are provided and doesn't mention the limitation
<braunr> 2/ when reading about UVM, one of the major improvements (between
10 and 30% of global performance depending on the benchmarks) was
implementing the madvise semantics
<braunr> and this didn't involve a new pager interface, but rather a new
page fault handler
<antrik> braunr: hm... the interface indeed looks like it can handle
multiple pages in both directions... perhaps it was at the Hurd level
where the pager interface needs to be modified, not the Mach one?...
<braunr> antrik: would be nice wouldn't it ? :)
<braunr> antrik: more probably the page fault handler
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