A long running Linux system uses 100% of its RAM. Every byte unused for applications will be used as a disk cache, given you read more data than your total RAM amount.
This cache is evictable, but it'll be there eventually.
Linux used to don't touch unused pages in the RAM in the older days if your RAM was not under pressure, but now it swaps out pages unused for a long time. This allows more cache space in RAM.
> how does caching to swap help?
I think I failed to convey what I tried to say. Let me retry:
Kernel doesn't cache to SSD. It swaps out unused (not accessed) but unevictable pages to SWAP, assuming that these pages will stay stale for a very long time, allowing more RAM to be used as cache.
When I look to my desktop system, in 12 days, Kernel moved 2592MB of my RAM to SWAP despite having ~20GB of free space. ~15GB of this free space is used as disk cache.
So, to have 2.5GB more disk cache, Kernel moved 2592 MB of non-accessed pages to SWAP.
This cache is evictable, but it'll be there eventually.
Linux used to don't touch unused pages in the RAM in the older days if your RAM was not under pressure, but now it swaps out pages unused for a long time. This allows more cache space in RAM.
> how does caching to swap help?
I think I failed to convey what I tried to say. Let me retry:
Kernel doesn't cache to SSD. It swaps out unused (not accessed) but unevictable pages to SWAP, assuming that these pages will stay stale for a very long time, allowing more RAM to be used as cache.
When I look to my desktop system, in 12 days, Kernel moved 2592MB of my RAM to SWAP despite having ~20GB of free space. ~15GB of this free space is used as disk cache.
So, to have 2.5GB more disk cache, Kernel moved 2592 MB of non-accessed pages to SWAP.