Re: Paging out when free memory is low but not exhausted (and available memory remains high)

[Chris Edwards <chris.edwards@otago.ac.nz>]

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Following the idea that it's some interaction with the X server, I further noticed that switching out of X to a text virtual console makes the page-outs stop. Going back to the X VT, the page-outs resume.

I've attached another set of vmstat logs for the following timeline:

1580290800	System is running Xorg, stress to limit memory, and dd to exercise the buffer cache, with constant page-outs
1580290810	Switch to a text virtual console - page-outs stop
1580290830	Switch back to X VT - page-outs resume

I'm vaguely suspecting something to do with the way Xorg handles old-fashioned programs that do CPU-driven bitmap-based rendering, as my desktop does typically have a lot of these (urxvt instances, xosview, the Notion window manager itself) - maybe they cause some particular pattern of memory churn in the X server, and perhaps only with certain video drivers...could Xorg perhaps wrongly madvise() the kernel about certain memory? It seems notable that having glxgears should cause the page-outs to stop.

However, even a minimal X session with a sakura or qterminal window seems to show some degree of needless page-outs with low memory and busy cache, though not as severe - however, it's difficult to avoid observer effects! There did seem to be a notable pattern of increasing swap utilisation when switching away from the X VT, and a drop in swap utilisation when switching back to X.

Should I perhaps take it up with the Xorg people instead?
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Chris

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