diff options
Diffstat (limited to 'hurd')
| -rw-r--r-- | hurd/translator/tmpfs/tmpfs_vs_defpager.mdwn | 115 |
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. |