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[[!tag open_issue_documentation open_issue_gnumach]]

[[!toc]]


# IRC, freenode, #hurd, 2011-02-15

    <braunr> etenil: originally, mach had its own virtual space (the kernel
      space)
    <braunr> etenil: in order to use linux 2.0 drivers, it now directly maps
      physical memory, as linux does
    <braunr> etenil: but there is nothing similar to kmap() or vmalloc() in
      mach, so the kernel is limited to its 1 GiB
    <braunr> (3 GiB userspace / 1 GiB kernelspace)
    <braunr> that's the short version, there is a vmalloc() in mach, but this
      trick made it behave almost like a kmalloc()
    <antrik> braunr: the direct mapping is *only* for the benefit of Linux
      drivers?...
    <braunr> also, the configuration of segments limits the kernel space
    <braunr> antrik: i'm not sure, as i said, this is the short version
    <braunr> antrik: but there is a paper which describes the integration of
      those drivers in mach
    <etenil> you mean the linux 2.0 drivers?
    <antrik> braunr: I read it once, but I don't remember anything about the
      physical mapping in there...
    <antrik> etenil: well, originally it was 1.3, but essentially that's the
      same...
    <braunr> i don't see any other reason why there would be a direct mapping
    <braunr> except for performance (because you can use larger - even very
      lage - pages without resetting the mmu often thanks to global pages, but
      that didn't exist at the time)


# IRC, freenode, #hurd, 2011-02-15

    <antrik> however, the kernel won't work in 64 bit mode without some changes
      to physical memory management
    <braunr> and mmu management
    <braunr> (but maybe that's what you meant by physical memory)


## IRC, freenode, #hurd, 2011-02-16

    <braunr> antrik: youpi added it for xen, yes
    <braunr> antrik: but you're right, since mach uses a direct mapped kernel
      space, the true problem is the lack of linux-like highmem support
    <braunr> which isn't required if the kernel space is really virtual


# IRC, freenode, #hurd, 2011-06-09

    <braunr> btw, how can gnumach use 1 GiB of RAM ? did you lower the
      user/kernel boundary address ?
    <youpi> I did
    <braunr> 2G ?
    <youpi> yes
    <braunr> ok
    <youpi> it doesn't make so much sense to let processes have 3G addressing
      space when there can't be more that 1G physical memory
    <braunr> that's sad for an operating system which does most things by
      mapping memory eh
    <youpi> well, if a process wants to map crazy things, 3G may be tight
      already
    <youpi> e.g. ext2fs
    <braunr> yes
    <youpi> so there's little point in supporting them
    <braunr> we need hurd/amd64
    <youpi> and there's quite some benefit in shrinking them to 2G
    <youpi> yes
    <youpi> actually even 2G may become a bit tight
    <youpi> webkit linking needs about 1.5-2GiB
    <youpi> things become really crazy
    <braunr> wow
    <braunr> i remember the linux support for 4G/4G split when there was enough
      RAM to fill the kernel space with struct page entries


# IRC, freenode, #hurd, 2011-11-12

    <youpi> well, the Hurd doesn't "artificially" limits itself to 1.5GiB
      memory
    <youpi> i386 has only 4GiB addressing space
    <youpi> we currently chose 2GiB for the kernel and 2GiB for the userspace
    <youpi> since kernel needs some mappings, that leaves only 1.5GiB usable
      physical memory
    <sea4ever`> Hm? 2GiB for kernel, 2GiB for userspace, 500MiB are used for
      what?
    <youpi> for mappings
    <youpi> such as device iomap
    <youpi> contiguous buffer allocation
    <youpi> and such things
    <sea4ever`> Ah, ok. You map things in kernel space into user space then.
    <youpi> linux does the same without the "bigmem" support
    <youpi> no, just in kernel space
    <youpi> kernel space is what determines how much physical memory you can
      address
    <youpi> unless using the linux-said-awful "bigmem" support


# IRC, freenode, #hurd, 2012-07-05

    <braunr> hm i got an address space exhaustion while building eglibc :/
    <braunr> we really need the 3/1 split back with a 64-bits kernel
    <pinotree> 3/1?
    <braunr> 3 GiB userspace, 1 GiB kernel
    <pinotree> ah
    <braunr> the debian gnumach package is patched to use a 2/2 split
    <braunr> and 2 GiB is really small for some needs
    <braunr> on the bright side, the machine didn't crash
    <braunr> there is issue with watch ./slabinfo which turned in a infinite
      loop, but it didn't affect the stability of the system
    <braunr> actually with a 64-bits kernel, we could use a 4/x split


# IRC, freenode, #hurd, 2012-08-10

    <braunr> all modern systems embed the kernel in every address space
    <braunr> which allows reduced overhead when making a system call
    <braunr> sometimes there is no context switch at all
    <braunr> on i386, there are security checks to upgrade the privilege level
      (switch to ring 0), and when used, kernel page tables are global, so
      they're not flushed
    <braunr> using sysenter/sysexit makes it even faster

[[service_solahart_jakarta_selatan__082122541663/system_call_mechanism]].


# IRC, freenode, #hurd, 2012-12-12

    <braunr> youpi: is the 2g split patch really needed ?
    <braunr> or rather, is it really a good thing for most people ?
    <braunr> instead of the common 3g/1g
    <braunr> it reduces tasks' address space but allows the kernel to reference
      more physical memory
    <braunr> the thing is, because of the current page cache implementation,
      most of the time, this physical memory remains unused, or very rarely
    <youpi> ?
    <braunr> on the other hand, a larger address space for tasks allows running
      more threads (more space for tasks) and not failing while linking webkit
      .. :)
    <youpi> it's needed for quite a few compilations, yes
    <braunr> if you refer to the link stage, with a decent amount of swap, it
      goes without trouble
    <youpi> well, if your kernel doesn't have 2GiB physical addressing
      capacity, userspace won't have >2GiB memory capacity either
    <youpi> does it now?
    <youpi> it didn't use to
    <youpi> and it was crawling like hell for some builds
    <youpi> (until simply hanging)
    <braunr> i never have a problem e.g. runing the big malloc glibc test
    <braunr> (bug22 or something like that)
    <youpi> that doesn't involve objects from the fs, does it?
    <braunr> no
    <braunr> as long as it's anonymous memory, it's ok
    <braunr> the default pager looks safe, i'm pretty sure our lockups are
      because of something in ext2fs
    <youpi> braunr: well, an alternative would be to build two kernels, one 2/2
      and one 3/1
    <braunr> not really worth it
    <braunr> i was just wondering
    <braunr> i usually prefer a 3/1 on darnassus, but i don't build as often as
      a buildd :x
    <youpi> or we can go with 2.5/1.5
    <youpi> I can do that on bach & mozart for instance
    <youpi> (they have their own kernel anyway)
    <braunr> youpi: if you think it's worth the effort
    <braunr> again, i was just wondering out loud
    <youpi> braunr: well, bach & mozart don't have > 1.2GiB mem anyway
    <youpi> so it doesn't pose problem


# IRC, freenode, #hurd, 2013-01-12

    <sobhan> can hurd have more than 1GB of ram ?
    <braunr> sobhan: not with the stock kernel, but yes with a simple patch
    <braunr> sobhan: although you should be aware of the implications of this
      patch
    <braunr> (more kernel memory, thus more physical memory - up to 1.8 GiB -
      but then, less user memory)


# IRC, freenode, #hurd, 2013-06-06

    <nlightnfotis> braunr: quick question, what memory allocation algorithms
      does the Mach use? I know it uses slab allocation, so I can guess buddy
      allocators too?
    <braunr> no
    <braunr> slab allocator for kernel memory (allocation of buffers used by
      the kernel itself)
    <braunr> a simple freelist for physical pages
    <braunr> and a custom allocator based on a red-black tree, a linked list
      and a hint for virtual memory
    <braunr> (which is practically the same in all BSD variants)
    <braunr> and linux does something very close too