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+[[!meta copyright="Copyright © 2011, 2014 Free Software Foundation, Inc."]]
+
+[[!meta license="""[[!toggle id="license" text="GFDL 1.2+"]][[!toggleable
+id="license" text="Permission is granted to copy, distribute and/or modify this
+document under the terms of the GNU Free Documentation License, Version 1.2 or
+any later version published by the Free Software Foundation; with no Invariant
+Sections, no Front-Cover Texts, and no Back-Cover Texts.  A copy of the license
+is included in the section entitled [[GNU Free Documentation
+License|/fdl]]."]]"""]]
+
+[[!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
+
+
+# IRC, freenode, #hurd, 2011-09-28
+
+    <slpz> antrik: I've just recovered part of my old multipage I/O work
+    <slpz> antrik: I intend to clean and submit it after finishing the changes
+      to the pageout system.
+    <antrik> slpz: oh, great!
+    <antrik> didn't know you worked on multipage I/O
+    <antrik> slpz: BTW, have you checked whether any of the work done for GSoC
+      last year is any good?...
+    <antrik> (apart from missing copyright assignments, which would be a
+      serious problem for the Hurd parts...)
+    <slpz> antrik: It was seven years ago, but I did:
+      http://www.mail-archive.com/bug-hurd@gnu.org/msg10285.html :-)
+    <slpz> antrik: Sincerely, I don't think the quality of that code is good
+      enough to be considered... but I think it was my fault as his mentor for
+      not correcting him soon enough...
+    <antrik> slpz: I see
+    <antrik> TBH, I feel guilty myself, for not asking about the situation
+      immediately when he stopped attending meetings...
+    <antrik> slpz: oh, you even already looked into vm_pageout_scan() back then
+      :-)
+
+
+# [[Read-Ahead]]