From be4193108513f02439a211a92fd80e0651f6721b Mon Sep 17 00:00:00 2001
From: Thomas Schwinge <tschwinge@gnu.org>
Date: Wed, 30 Nov 2011 21:21:45 +0100
Subject: IRC.
---
hurd/translator/tmpfs/tmpfs_vs_defpager.mdwn | 37 ++++++++++++++++++++++++++++
1 file changed, 37 insertions(+)
(limited to 'hurd/translator/tmpfs')
diff --git a/hurd/translator/tmpfs/tmpfs_vs_defpager.mdwn b/hurd/translator/tmpfs/tmpfs_vs_defpager.mdwn
index a9317c21..5228515f 100644
--- a/hurd/translator/tmpfs/tmpfs_vs_defpager.mdwn
+++ b/hurd/translator/tmpfs/tmpfs_vs_defpager.mdwn
@@ -233,3 +233,40 @@ See also: [[open_issues/resource_management_problems/pagers]].
I have never looked at it.
[[open_issues/mach-defpager_vs_defpager]].
+
+
+# IRC, freenode, #hurd, 2011-11-08
+
+ <mcsim> who else uses defpager besides tmpfs and kernel?
+ <braunr> normally, nothing directly
+ <mcsim> than why tmpfs should use defpager?
+ <braunr> it's its backend
+ <braunr> backign store rather
+ <braunr> the backing store of most file systems are partitions
+ <braunr> tmpfs has none, it uses the swap space
+ <mcsim> if we allocate memory for tmpfs using vm_allocate, will it be able
+ to use swap partition?
+ <braunr> it should
+ <braunr> vm_allocate just maps anonymous memory
+ <braunr> anonymous memory uses swap space as its backing store too
+ <braunr> but be aware that this part of the vm system is known to have
+ deficiencies
+ <braunr> which is why all mach based implementations have rewritten their
+ default pager
+ <mcsim> what kind of deficiencies?
+ <braunr> bugs
+ <braunr> and design issues, making anonymous memory fragmentation horrible
+ <antrik> mcsim: vm_allocate doesn't return a memory object; so it can't be
+ passed to clients for mmap()
+ <mcsim> antrik: I use vm_allocate in pager_read_page
+ <antrik> mcsim: well, that means that you have to actually implement a
+ pager yourself
+ <antrik> also, when the kernel asks the pager to write back some pages, it
+ expects the memory to become free. if you are "paging" to ordinary
+ anonymous memory, this doesn't happen; so I expect it to have a very bad
+ effect on system performance
+ <antrik> both can be avoided by just passing a real anonymous memory
+ object, i.e. one provided by the defpager
+ <antrik> only problem is that the current defpager implementation can't
+ really handle that...
+ <antrik> at least that's my understanding of the situation
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