ia64/linux-2.6.18-xen.hg

view Documentation/vm/balance @ 854:950b9eb27661

usbback: fix urb interval value for interrupt urbs.

Signed-off-by: Noboru Iwamatsu <n_iwamatsu@jp.fujitsu.com>
author Keir Fraser <keir.fraser@citrix.com>
date Mon Apr 06 13:51:20 2009 +0100 (2009-04-06)
parents 831230e53067
children
line source
1 Started Jan 2000 by Kanoj Sarcar <kanoj@sgi.com>
3 Memory balancing is needed for non __GFP_WAIT as well as for non
4 __GFP_IO allocations.
6 There are two reasons to be requesting non __GFP_WAIT allocations:
7 the caller can not sleep (typically intr context), or does not want
8 to incur cost overheads of page stealing and possible swap io for
9 whatever reasons.
11 __GFP_IO allocation requests are made to prevent file system deadlocks.
13 In the absence of non sleepable allocation requests, it seems detrimental
14 to be doing balancing. Page reclamation can be kicked off lazily, that
15 is, only when needed (aka zone free memory is 0), instead of making it
16 a proactive process.
18 That being said, the kernel should try to fulfill requests for direct
19 mapped pages from the direct mapped pool, instead of falling back on
20 the dma pool, so as to keep the dma pool filled for dma requests (atomic
21 or not). A similar argument applies to highmem and direct mapped pages.
22 OTOH, if there is a lot of free dma pages, it is preferable to satisfy
23 regular memory requests by allocating one from the dma pool, instead
24 of incurring the overhead of regular zone balancing.
26 In 2.2, memory balancing/page reclamation would kick off only when the
27 _total_ number of free pages fell below 1/64 th of total memory. With the
28 right ratio of dma and regular memory, it is quite possible that balancing
29 would not be done even when the dma zone was completely empty. 2.2 has
30 been running production machines of varying memory sizes, and seems to be
31 doing fine even with the presence of this problem. In 2.3, due to
32 HIGHMEM, this problem is aggravated.
34 In 2.3, zone balancing can be done in one of two ways: depending on the
35 zone size (and possibly of the size of lower class zones), we can decide
36 at init time how many free pages we should aim for while balancing any
37 zone. The good part is, while balancing, we do not need to look at sizes
38 of lower class zones, the bad part is, we might do too frequent balancing
39 due to ignoring possibly lower usage in the lower class zones. Also,
40 with a slight change in the allocation routine, it is possible to reduce
41 the memclass() macro to be a simple equality.
43 Another possible solution is that we balance only when the free memory
44 of a zone _and_ all its lower class zones falls below 1/64th of the
45 total memory in the zone and its lower class zones. This fixes the 2.2
46 balancing problem, and stays as close to 2.2 behavior as possible. Also,
47 the balancing algorithm works the same way on the various architectures,
48 which have different numbers and types of zones. If we wanted to get
49 fancy, we could assign different weights to free pages in different
50 zones in the future.
52 Note that if the size of the regular zone is huge compared to dma zone,
53 it becomes less significant to consider the free dma pages while
54 deciding whether to balance the regular zone. The first solution
55 becomes more attractive then.
57 The appended patch implements the second solution. It also "fixes" two
58 problems: first, kswapd is woken up as in 2.2 on low memory conditions
59 for non-sleepable allocations. Second, the HIGHMEM zone is also balanced,
60 so as to give a fighting chance for replace_with_highmem() to get a
61 HIGHMEM page, as well as to ensure that HIGHMEM allocations do not
62 fall back into regular zone. This also makes sure that HIGHMEM pages
63 are not leaked (for example, in situations where a HIGHMEM page is in
64 the swapcache but is not being used by anyone)
66 kswapd also needs to know about the zones it should balance. kswapd is
67 primarily needed in a situation where balancing can not be done,
68 probably because all allocation requests are coming from intr context
69 and all process contexts are sleeping. For 2.3, kswapd does not really
70 need to balance the highmem zone, since intr context does not request
71 highmem pages. kswapd looks at the zone_wake_kswapd field in the zone
72 structure to decide whether a zone needs balancing.
74 Page stealing from process memory and shm is done if stealing the page would
75 alleviate memory pressure on any zone in the page's node that has fallen below
76 its watermark.
78 pages_min/pages_low/pages_high/low_on_memory/zone_wake_kswapd: These are
79 per-zone fields, used to determine when a zone needs to be balanced. When
80 the number of pages falls below pages_min, the hysteric field low_on_memory
81 gets set. This stays set till the number of free pages becomes pages_high.
82 When low_on_memory is set, page allocation requests will try to free some
83 pages in the zone (providing GFP_WAIT is set in the request). Orthogonal
84 to this, is the decision to poke kswapd to free some zone pages. That
85 decision is not hysteresis based, and is done when the number of free
86 pages is below pages_low; in which case zone_wake_kswapd is also set.
89 (Good) Ideas that I have heard:
90 1. Dynamic experience should influence balancing: number of failed requests
91 for a zone can be tracked and fed into the balancing scheme (jalvo@mbay.net)
92 2. Implement a replace_with_highmem()-like replace_with_regular() to preserve
93 dma pages. (lkd@tantalophile.demon.co.uk)