Commit | Line | Data |
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1da177e4 LT |
1 | Started Jan 2000 by Kanoj Sarcar <kanoj@sgi.com> |
2 | ||
3 | Memory balancing is needed for non __GFP_WAIT as well as for non | |
4 | __GFP_IO allocations. | |
5 | ||
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. | |
10 | ||
11 | __GFP_IO allocation requests are made to prevent file system deadlocks. | |
12 | ||
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. | |
17 | ||
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. | |
25 | ||
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. | |
33 | ||
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. | |
42 | ||
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. | |
51 | ||
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. | |
56 | ||
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) | |
65 | ||
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. | |
73 | ||
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. | |
77 | ||
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. | |
87 | ||
88 | ||
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) |