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why SUnreclaim in slab cache grow extremely high but can be greatly free up through drop_caches?

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We observe extremely high slab memory usage on a long running CentOS7(mount Lustre) server. With total 187G RAM, the slab cache used 110+G and most part is SUnreclaim.

Since slab = SUnreclaim + SReclaimable, we think the SUnreclaim memory should be actually used and cannot be free up. But after executing echo 2 > /proc/sys/vm/drop_caches, we found the SUnreclaim size also be greatly decreased. The detail memory info shown below:

enter image description here

So why SUnreclaim can also be greatly free up? If so, what is the meaning to split slab into SUnreclaim and SReclaimable?

And when we want to monitor the memory usage, how should we calculate the actually used memory in the case where SUnreclaim can eat such high memory?

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