Hello. When copy files to a USB device, I see the following messages in the Syslog:
KERN: low resource pages: note -> normal
KERN: low resource pages: normal -> note
That’s entries repeat several times. What they mean?
Thank you!
Hi,
This happens because writing to USB devices is slower than reading from your source drive. The result of this is that a lot of data is read from the source, and put into the file cache. The writing side doesn't empty the file cache fast enough, and as a result of this the file cache starts to grow and use more RAM.
After some time, the kernel notices that the file cache is using a lot of memory pages, and swiches from "normal" to "note". This triggers the "low resource manager", which will ask all kernel services to try to free some pages. As soon as this is done, the situation goes back to "normal".
It is nothing to worry about unless you see it switching to "warning" or "critical" state. This would mean the kernel has very few pages left and is struggling to keep things running.
The plan to fix this is to move the file cache out of the kernel page system. It should use something similar to the block cache, that is, allocating physical memory pages without mapping them in the kernel address space directly. Basically this means the file cache would use "free RAM" for caching. It would need to flush things to disk once that RAM needs to be allocated for other uses, but at least it wouldn't use pages as far as the kernel accounting is concerned. This is one of our GSoC ideas (it has been for several years).
Thank you for the explanation. Very clear and didactic.
Thank you Pulkomandy!!!