From bdd896e0b81cfb40c8d24a78f9022f6cd1ae5e8c Mon Sep 17 00:00:00 2001 From: Thomas Schwinge Date: Thu, 17 Feb 2011 14:15:11 +0100 Subject: open_issues/performance/io_system/clustered_page_faults: New. And some more IRC discussions. --- .../io_system/clustered_page_faults.mdwn | 103 +++++++++++++++++++++ 1 file changed, 103 insertions(+) create mode 100644 open_issues/performance/io_system/clustered_page_faults.mdwn (limited to 'open_issues/performance/io_system/clustered_page_faults.mdwn') diff --git a/open_issues/performance/io_system/clustered_page_faults.mdwn b/open_issues/performance/io_system/clustered_page_faults.mdwn new file mode 100644 index 00000000..3a187523 --- /dev/null +++ b/open_issues/performance/io_system/clustered_page_faults.mdwn @@ -0,0 +1,103 @@ +[[!meta copyright="Copyright © 2011 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]] + +IRC, freenode, #hurd, 2011-02-16 + + exceptfor the kernel, everything in an address space is + represented with a VM object + those objects can represent anonymous memory (from malloc() or + because of a copy-on-write) + or files + on classic Unix systems, these are files + on the Hurd, these are memory objects, backed by external pagers + (like ext2fs) + so when you read a file + the kernel maps it from ext2fs in your address space + and when you access the memory, a fault occurs + the kernel determines it's a region backed by ext2fs + so it asks ext2fs to provide the data + when the fault is resolved, your process goes on + does the faul occur because Mach doesn't know how to access the + memory? + it occurs because Mach intentionnaly didn't back the region with + physical memory + the MMU is programmed not to know what is present in the memory + region + or because it's read only + (which is the case for COW faults) + 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 + more or less, yes + ideally, it's directly written into the right pages + there is no intermediate buffer + I see + and as you told me before, currently the page faults are handled + one at a time + which wastes a lot of time + a certain amount of time + enough to bother the user :) + I've seen pages have a fixed size + yes + use the PAGE_SIZE macro + and when allocating memory, the size that's asked for is rounded + up to the page size + so if I have this correctly, it means that a file ext2fs provides + could be split into a lot of pages + yes + once in memory, it is managed by the page cache + so that pages more actively used are kept longer than others + in order to minimize I/O + ok + so a better page cache code would also improve overall performance + and more RAM would help a lot, since we are strongly limited by + the 768 MiB limit + which reduces the page cache size a lot + but the problem is that reading a whole file in means trigerring + many page faults just for one file + if you want to stick to the page clustering thing, yes + you want less page faults, so that there are less IPC between the + kernel and the pager + so either I make pages bigger + or I modify Mach so it can check up on a range of pages for faults + before actually processing + you *don't* change the page size + ah + that's hardware isn't it? + in Mach, yes + ok + and usually, you want the page size to be the CPU page size + I see + current CPU can support multiple page sizes, but it becomes quite + hard to correctly handle + and bigger page sizes mean more fragmentation, so it only suits + machines with large amounts of RAM, which isn't the case for us + ok + so I'll try the second approach then + that's what i'd recommand + recommend* + ok + +--- + +IRC, freenode, #hurd, 2011-02-16 + + etenil: OSF Mach does have clustered paging BTW; so that's one + place to start looking... + (KAM ported the OSF code to gnumach IIRC) + there is also an existing patch for clustered paging in libpager, + which needs some adaptation + the biggest part of the task is probably modifying the Hurd + servers to use the new interface + but as I said, KAM's code should be available through google, and + can serve as a starting point + + -- cgit v1.2.3