Messages in this thread | | | Date | Sat, 10 Sep 2005 10:50:11 +1000 | From | Anthony Wesley <> | Subject | Re: kernel 2.6.13 buffer strangeness |
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Based on a better understanding of dirty_ratio and dirty_background ratio (thanks Nate) I just tried the following test:
dirty_ratio set to 95 dirty_background_ratio set to 1
From Nate's description of these parameters, this should mean that the disk writes start almost immediately, and the kernel will allow 95% of RAM to become dirty before applying the throttle.
Ok, so with 25Mbytes/s coming in, and 17Mbytes/sec going out to disk, the dirty pages should be growing at 7Mbytes/sec. With these parameters set as above I should see about 3 minutes of full speed video before the throttle is applied since I have about 1.3Gb of RAM free for buffering..
*But* when I try this experiment I hit the throttle after only 65 seconds - an improvement to be sure, but still a long way short of the 180 seconds that it ought to take.
Part of the test works as expected - the disk writes begin almost immediately due to the low value for dirty_background_ratio, but the rest is a mystery.
It really looks as if the pages aren't being marked as clean fast enough after they're written.
How else can it take only 70 seconds to reach 95% dirty when I have 1.3Gb of available RAM and data coming in at 25MBytes/sec and out at 17MBytes/sec? It doesn't make any sense...
regards, Anthony
Nate Diller wrote: > yes, on 2.6 there are two tunables which are important here. > dirty_background_ratio is the threshold where the kernel will begin > flushing dirty buffers, so it should change how soon the disk becomes > active. dirty_ratio changes when the write-throttling code kicks in, > which is what Anthony is seeing. The purpose of the write throttling > code is to limit the dirtying process to disk bandwidth, so that is a > Feature. Anthony, try *increasing* dirty_ratio, you can go up to 100, > but you could trigger an OOM if you let it get too high, so maybe try > setting it at 85 or so. This should effectively disable the write > throttling and give you the bandwidth you want. > > NATE
-- Anthony Wesley Director and IT/Network Consultant Smart Networks Pty Ltd Acquerra Pty Ltd
Anthony.Wesley@acquerra.com.au Phone: (02) 62595404 or 0419409836 - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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