94 lines
5 KiB
Text
94 lines
5 KiB
Text
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Started Jan 2000 by Kanoj Sarcar <kanoj@sgi.com>
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Memory balancing is needed for non __GFP_WAIT as well as for non
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__GFP_IO allocations.
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There are two reasons to be requesting non __GFP_WAIT allocations:
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the caller can not sleep (typically intr context), or does not want
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to incur cost overheads of page stealing and possible swap io for
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whatever reasons.
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__GFP_IO allocation requests are made to prevent file system deadlocks.
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In the absence of non sleepable allocation requests, it seems detrimental
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to be doing balancing. Page reclamation can be kicked off lazily, that
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is, only when needed (aka zone free memory is 0), instead of making it
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a proactive process.
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That being said, the kernel should try to fulfill requests for direct
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mapped pages from the direct mapped pool, instead of falling back on
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the dma pool, so as to keep the dma pool filled for dma requests (atomic
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or not). A similar argument applies to highmem and direct mapped pages.
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OTOH, if there is a lot of free dma pages, it is preferable to satisfy
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regular memory requests by allocating one from the dma pool, instead
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of incurring the overhead of regular zone balancing.
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In 2.2, memory balancing/page reclamation would kick off only when the
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_total_ number of free pages fell below 1/64 th of total memory. With the
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right ratio of dma and regular memory, it is quite possible that balancing
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would not be done even when the dma zone was completely empty. 2.2 has
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been running production machines of varying memory sizes, and seems to be
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doing fine even with the presence of this problem. In 2.3, due to
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HIGHMEM, this problem is aggravated.
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In 2.3, zone balancing can be done in one of two ways: depending on the
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zone size (and possibly of the size of lower class zones), we can decide
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at init time how many free pages we should aim for while balancing any
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zone. The good part is, while balancing, we do not need to look at sizes
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of lower class zones, the bad part is, we might do too frequent balancing
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due to ignoring possibly lower usage in the lower class zones. Also,
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with a slight change in the allocation routine, it is possible to reduce
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the memclass() macro to be a simple equality.
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Another possible solution is that we balance only when the free memory
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of a zone _and_ all its lower class zones falls below 1/64th of the
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total memory in the zone and its lower class zones. This fixes the 2.2
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balancing problem, and stays as close to 2.2 behavior as possible. Also,
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the balancing algorithm works the same way on the various architectures,
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which have different numbers and types of zones. If we wanted to get
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fancy, we could assign different weights to free pages in different
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zones in the future.
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Note that if the size of the regular zone is huge compared to dma zone,
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it becomes less significant to consider the free dma pages while
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deciding whether to balance the regular zone. The first solution
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becomes more attractive then.
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The appended patch implements the second solution. It also "fixes" two
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problems: first, kswapd is woken up as in 2.2 on low memory conditions
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for non-sleepable allocations. Second, the HIGHMEM zone is also balanced,
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so as to give a fighting chance for replace_with_highmem() to get a
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HIGHMEM page, as well as to ensure that HIGHMEM allocations do not
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fall back into regular zone. This also makes sure that HIGHMEM pages
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are not leaked (for example, in situations where a HIGHMEM page is in
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the swapcache but is not being used by anyone)
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kswapd also needs to know about the zones it should balance. kswapd is
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primarily needed in a situation where balancing can not be done,
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probably because all allocation requests are coming from intr context
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and all process contexts are sleeping. For 2.3, kswapd does not really
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need to balance the highmem zone, since intr context does not request
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highmem pages. kswapd looks at the zone_wake_kswapd field in the zone
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structure to decide whether a zone needs balancing.
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Page stealing from process memory and shm is done if stealing the page would
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alleviate memory pressure on any zone in the page's node that has fallen below
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its watermark.
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watemark[WMARK_MIN/WMARK_LOW/WMARK_HIGH]/low_on_memory/zone_wake_kswapd: These
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are per-zone fields, used to determine when a zone needs to be balanced. When
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the number of pages falls below watermark[WMARK_MIN], the hysteric field
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low_on_memory gets set. This stays set till the number of free pages becomes
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watermark[WMARK_HIGH]. When low_on_memory is set, page allocation requests will
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try to free some pages in the zone (providing GFP_WAIT is set in the request).
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Orthogonal to this, is the decision to poke kswapd to free some zone pages.
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That decision is not hysteresis based, and is done when the number of free
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pages is below watermark[WMARK_LOW]; in which case zone_wake_kswapd is also set.
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(Good) Ideas that I have heard:
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1. Dynamic experience should influence balancing: number of failed requests
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for a zone can be tracked and fed into the balancing scheme (jalvo@mbay.net)
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2. Implement a replace_with_highmem()-like replace_with_regular() to preserve
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dma pages. (lkd@tantalophile.demon.co.uk)
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