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 Date Fri, 07 Sep 2001 10:15:27 +0100 From Alex Bligh - linux-kernel <> Subject Re: [RFC] Defragmentation proposal: preventative maintenance and cleanup [LONG]
`>> It becomes effectively useless.  The probability of all 8 pages of a given>> 8 page unit being free when only 1% of memory is free is (1/100)**8 =>> 1/(10**16).> Sorry to sound like a broken record, but apply the> /proc/memareas patch and you can see this happening. After extensive> activity, you see practically none of the free pages in order 0> blocks. You might see only a small number (20 or 30 on a 64k> machine) of (say) order 3 blocks, but if you run your stats> you would have an expected value of well less than one, and the> chance of having 20 or 30 would be vanishingly small.Ooops, what I wrote was factually correct, but misleading.What I meant was it looks like this:   Zone     4kB     8kB    16kB    32kB    64kB   128kB   256kB   512kB  1024kB  2048kB Tot Pages/kb    DMA     495     348     196      72      10       1       1       0       0       0 =     2807)  @frag      0%     18%     42%     70%     91%     97%     98%    100%    100%    100% =    11228kB Normal       0    1579    1670     667     140      12       3       1       0       0 =    18118)  @frag      0%      0%     17%     54%     84%     96%     98%     99%    100%    100% =    72472kBIf your model was correct, you would see free pagesper order run like  N = a (K ^ (2^-o)); (for a>0, K>1, o=order)This doesn't happen. Instead you get GOOD coalescenceat oder 0 (in the Normal zone they've ALL been coalesced),and not bad at order 1 (see how many order 2's we have).8 page unit is order 3 (32k). This system has 20% ofmemory free at the point where I took the snap shot.Probability would be, (1/5)^8 = 2^8 / 10^8 =roughly p = 2.5 x 10^-6. In a system with 32000 pages(128Kb), if you were right, I'd expect to see about0.08 free pages at order 3. But here I see 750.The chance of seeing more than 500 events of probabilityp = 2.5 ^ (10^-6) across 32000 samples, is vanishinglysmall. Yet it looks this way all the time.Hence I conclude your model is wrong :-)--Alex Bligh-To unsubscribe from this list: send the line "unsubscribe linux-kernel" inthe body of a message to majordomo@vger.kernel.orgMore majordomo info at  http://vger.kernel.org/majordomo-info.htmlPlease read the FAQ at  http://www.tux.org/lkml/`

Last update: 2005-03-22 13:02    [from the cache]