Messages in this thread | | | Subject | bdi_threshold slow to reach steady state | From | Richard Kennedy <> | Date | Wed, 14 Oct 2009 12:09:45 +0100 |
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Hi Peter,
I've been running simple tests that uses fio to write 2Gb & reading the bdi dirty threshold once a second from debugfs.
The graph of bdi dirty threshold is nice and smooth but takes a long time to reach a steady state, 60 seconds or more. (run on 2.6.32-rc4)
By eye it seems as though a first-order control system is a good model for its behavior, so it approximates to 1-e^(-t/T). It just seems too heavily damped ( at least on my machine).
For fun, I changed calc_period_shift to return ilog2(dirty_total - 1) - 2;
and it now reaches a steady state much quicker, around 4-5 seconds.
Tests that write to 2 disks at the same time show no significant performance differences but are much more consistent, i.e. the standard deviation is lower across multiple runs.
I have noticed that the first test run on a freshly booted machine is always the slowest of any sequence of tests, but this change to calc_period_shift greatly reduces this effect.
So I wondered how you chose these values? and are there any other tests that are useful to explore this?
I know that my machine is getting a bit old now, it's AMDX2 & only has sata 150 drives, so I'm not suggesting that this change is going to be correct for all machines but maybe we can set a better default? or take more factors in to account other than just memory size.
BTW why is it ilog2(dirty_total -1) -- what does the -1 do?
regards Richard
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