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-rw-r--r--hurd/translator/tmpfs/tmpfs_vs_defpager.mdwn115
1 files changed, 113 insertions, 2 deletions
diff --git a/hurd/translator/tmpfs/tmpfs_vs_defpager.mdwn b/hurd/translator/tmpfs/tmpfs_vs_defpager.mdwn
index f0eb473c..ecebe662 100644
--- a/hurd/translator/tmpfs/tmpfs_vs_defpager.mdwn
+++ b/hurd/translator/tmpfs/tmpfs_vs_defpager.mdwn
@@ -8,9 +8,10 @@ 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_hurd]]
+[[!tag open_issue_gnumach open_issue_hurd]]
-\#hurd, freenode, 2010
+
+# IRC, freenode, #hurd, 2010
<slpz> humm... why does tmpfs try to use the default pager? that's a bad
idea, and probably will never work correctly...
@@ -120,3 +121,113 @@ License|/fdl]]."]]"""]]
memory, gives them a reference to the default pager by calling
vm_object_pager_create
<slpz> this is not really important, but worth noting ;-)
+
+
+# IRC, freenode, #hurd, 2011-09-28
+
+ <slpz> mcsim: "Fix tmpfs" task should be called "Fix default pager" :-)
+ <slpz> mcsim: I've been thinking about modifying tmpfs to actually have
+ it's own storeio based backend, even if a tmpfs with storage sounds a bit
+ stupid.
+ <slpz> mcsim: but I don't like the idea of having translators messing up
+ with the default pager...
+ <antrik> slpz: messing up?...
+ <slpz> antrik: in the sense of creating a number of arbitrarily sized
+ objects
+ <antrik> slpz: well, it doesn't really matter much whether a process
+ indirectly eats up arbitrary amounts of swap through tmpfs, or directly
+ through vm_allocate()...
+ <antrik> though admittedly it's harder to implement resource limits with
+ tmpfs
+ <slpz> antrik: but I've talked about having its own storeio device as
+ backend. This way Mach can pageout memory to tmpfs if it's needed.
+ <mcsim> Do I understand correctly that the goal of tmpfs task is to create
+ tmpfs in RAM?
+ <slpz> mcsim: It is. But it also needs some kind of backend, just in case
+ it's ordered to page out data to free some system's memory.
+ <slpz> mcsim: Nowadays, this backend is another translator that acts as
+ default pager for the whole system
+ <antrik> slpz: pageout memory to tmpfs? not sure what you mean
+ <slpz> antrik: I mean tmpfs acting as its own pager
+ <antrik> slpz: you mean tmpfs not using the swap partition, but some other
+ backing store?
+ <slpz> antrik: Yes.
+
+See also: [[open_issues/resource_management_problems/pagers]].
+
+ <antrik> slpz: I don't think an extra backing store for tmpfs is a good
+ idea. the whole point of tmpfs is not having a backing store... TBH, I'd
+ even like to see a single backing store for anonymous memory and named
+ files
+ <slpz> antrik: But you need a backing store, even if it's the default pager
+ :-)
+ <slpz> antrik: The question is, Should users share the same backing store
+ (swap space) or provide their own?
+ <antrik> slpz: not sure what you mean by "users" in this context :-)
+ <slpz> antrik: Real users with the ability of setting tmpfs translators
+ <antrik> essentially, I'd like to have a single partition that contains
+ both swap space and the main filesystem (at least /tmp, but probably also
+ all of /run, and possibly even /home...)
+ <antrik> but that's a bit off-topic :-)
+ <antrik> well, ideally all storage should be accounted to a user,
+ regardless whether it's swapped out anonymous storage, temporary named
+ files, or permanent files
+ <slpz> antrik: you could use a file as backend for tmpfs
+ <antrik> slpz: what's the point of using tmpfs then? :-)
+ <pinotree> (and then store the file in another tmpfs)
+ <slpz> antrik: mach-defpager could be modified to use storeio instead of
+ Mach's device_* operations, but by the way things work right now, that
+ could be dangerous, IMHO
+ <antrik> pinotree: hehe
+ <pinotree> .. recursive tmpfs'es ;)
+ <antrik> slpz: hm, sounds interesting
+ <slpz> antrik: tmpfs would try to keep data in memory always it's possible
+ (not calling m_o_lock_request would do the trick), but if memory is
+ scarce an Mach starts paging out, it would write it to that
+ file/device/whatever
+ <antrik> ideally, all storage used by system tasks for swapped out
+ anonymous memory as well as temporary named files would end up on the
+ /run partition; while all storage used by users would end up in /home/*
+ <antrik> if users share a partition, some explicit storage accounting would
+ be useful too...
+ <antrik> slpz: is that any different from what "normal" filesystems do?...
+ <antrik> (and *should* it be different?...)
+ <slpz> antrik: Yes, as most FS try to synchronize to disk at a reasonable
+ rate, to prevent data losses.
+ <slpz> antrik: tmpfs would be a FS that wouldn't synchronize until it's
+ forced to do that (which, by the way, it's what's currently happening
+ with everyone that uses the default pager).
+ <antrik> slpz: hm, good point...
+ <slpz> antrik: Also, metadata in never written to disk, only kept in memory
+ (which saves a lot of I/O, too).
+ <slpz> antrik: In fact, we would be doing the same as every other kernel
+ does, but doing it explicitly :-)
+ <antrik> I see the use in separating precious data (in permanent named
+ files) from temporary state (anonymous memory and temporary named files)
+ -- but I'm not sure whether having a completely separate FS for the
+ temporary data is the right approach for that...
+ <slpz> antrik: And giving the user the option to specify its own storage,
+ so we don't limit him to the size established for swap by the super-user.
+ <antrik> either way, that would be a rather radical change... still would
+ be good to fix tmpfs as it is first if possible
+ <antrik> as for limited swap, that's precisely why I'd prefer not to have
+ an extra swap partition at all...
+ <slpz> antrik: It's not much o fa change, it's how it works right now, with
+ the exception of replacing the default pager with its own.
+ <slpz> antrik: I think it's just a matter of 10-20 hours, as
+ much. Including testing.
+ <slpz> antrik: It could be forked with another name, though :-)
+ <antrik> slpz: I don't mean radical change in the implementation... but a
+ radical change in the way it would be used
+ <slpz> antrik: I suggest "almosttmpfs" as the name for the forked one :-P
+ <antrik> hehe
+ <antrik> how about lazyfs?
+ <slpz> antrik: That sound good to me, but probably we should use a more
+ descriptive name :-)
+
+
+## 2011-09-29
+
+ <tschwinge> slpz, antrik: There is a defpager in the Hurd code. It is not
+ currently being used, and likely incomplete. It is backed by libstore.
+ I have never looked at it.