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-Started Jan 2000 by Kanoj Sarcar <>
-Memory balancing is needed for non __GFP_WAIT as well as for non
-__GFP_IO allocations.
-There are two reasons to be requesting non __GFP_WAIT allocations:
-the caller can not sleep (typically intr context), or does not want
-to incur cost overheads of page stealing and possible swap io for
-whatever reasons.
-__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 (
-2. Implement a replace_with_highmem()-like replace_with_regular() to preserve
-dma pages. (