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Diffstat (limited to 'Documentation/vm/balance')
| -rw-r--r-- | Documentation/vm/balance | 95 |
1 files changed, 0 insertions, 95 deletions
diff --git a/Documentation/vm/balance b/Documentation/vm/balance deleted file mode 100644 index 964595481af6..000000000000 --- a/Documentation/vm/balance +++ /dev/null @@ -1,95 +0,0 @@ -Started Jan 2000 by Kanoj Sarcar <kanoj@sgi.com> - -Memory balancing is needed for !__GFP_ATOMIC and !__GFP_KSWAPD_RECLAIM as -well as for non __GFP_IO allocations. - -The first reason why a caller may avoid reclaim is that the caller can not -sleep due to holding a spinlock or is in interrupt context. The second may -be that the caller is willing to fail the allocation without incurring the -overhead of page reclaim. This may happen for opportunistic high-order -allocation requests that have order-0 fallback options. In such cases, -the caller may also wish to avoid waking kswapd. - -__GFP_IO allocation requests are made to prevent file system deadlocks. - -In the absence of non sleepable allocation requests, it seems detrimental -to be doing balancing. Page reclamation can be kicked off lazily, that -is, only when needed (aka zone free memory is 0), instead of making it -a proactive process. - -That being said, the kernel should try to fulfill requests for direct -mapped pages from the direct mapped pool, instead of falling back on -the dma pool, so as to keep the dma pool filled for dma requests (atomic -or not). A similar argument applies to highmem and direct mapped pages. -OTOH, if there is a lot of free dma pages, it is preferable to satisfy -regular memory requests by allocating one from the dma pool, instead -of incurring the overhead of regular zone balancing. - -In 2.2, memory balancing/page reclamation would kick off only when the -_total_ number of free pages fell below 1/64 th of total memory. With the -right ratio of dma and regular memory, it is quite possible that balancing -would not be done even when the dma zone was completely empty. 2.2 has -been running production machines of varying memory sizes, and seems to be -doing fine even with the presence of this problem. In 2.3, due to -HIGHMEM, this problem is aggravated. - -In 2.3, zone balancing can be done in one of two ways: depending on the -zone size (and possibly of the size of lower class zones), we can decide -at init time how many free pages we should aim for while balancing any -zone. The good part is, while balancing, we do not need to look at sizes -of lower class zones, the bad part is, we might do too frequent balancing -due to ignoring possibly lower usage in the lower class zones. Also, -with a slight change in the allocation routine, it is possible to reduce -the memclass() macro to be a simple equality. - -Another possible solution is that we balance only when the free memory -of a zone _and_ all its lower class zones falls below 1/64th of the -total memory in the zone and its lower class zones. This fixes the 2.2 -balancing problem, and stays as close to 2.2 behavior as possible. Also, -the balancing algorithm works the same way on the various architectures, -which have different numbers and types of zones. If we wanted to get -fancy, we could assign different weights to free pages in different -zones in the future. - -Note that if the size of the regular zone is huge compared to dma zone, -it becomes less significant to consider the free dma pages while -deciding whether to balance the regular zone. The first solution -becomes more attractive then. - -The appended patch implements the second solution. It also "fixes" two -problems: first, kswapd is woken up as in 2.2 on low memory conditions -for non-sleepable allocations. Second, the HIGHMEM zone is also balanced, -so as to give a fighting chance for replace_with_highmem() to get a -HIGHMEM page, as well as to ensure that HIGHMEM allocations do not -fall back into regular zone. This also makes sure that HIGHMEM pages -are not leaked (for example, in situations where a HIGHMEM page is in -the swapcache but is not being used by anyone) - -kswapd also needs to know about the zones it should balance. kswapd is -primarily needed in a situation where balancing can not be done, -probably because all allocation requests are coming from intr context -and all process contexts are sleeping. For 2.3, kswapd does not really -need to balance the highmem zone, since intr context does not request -highmem pages. kswapd looks at the zone_wake_kswapd field in the zone -structure to decide whether a zone needs balancing. - -Page stealing from process memory and shm is done if stealing the page would -alleviate memory pressure on any zone in the page's node that has fallen below -its watermark. - -watemark[WMARK_MIN/WMARK_LOW/WMARK_HIGH]/low_on_memory/zone_wake_kswapd: These -are per-zone fields, used to determine when a zone needs to be balanced. When -the number of pages falls below watermark[WMARK_MIN], the hysteric field -low_on_memory gets set. This stays set till the number of free pages becomes -watermark[WMARK_HIGH]. When low_on_memory is set, page allocation requests will -try to free some pages in the zone (providing GFP_WAIT is set in the request). -Orthogonal to this, is the decision to poke kswapd to free some zone pages. -That decision is not hysteresis based, and is done when the number of free -pages is below watermark[WMARK_LOW]; in which case zone_wake_kswapd is also set. - - -(Good) Ideas that I have heard: -1. Dynamic experience should influence balancing: number of failed requests -for a zone can be tracked and fed into the balancing scheme (jalvo@mbay.net) -2. Implement a replace_with_highmem()-like replace_with_regular() to preserve -dma pages. (lkd@tantalophile.demon.co.uk) |