What is kswapd_low_wmark_hit_quickly from /proc/vmstat

Question

What does this parameter mean?

# cat /proc/vmstat | grep low
kswapd_low_wmark_hit_quickly 2

Answer 1

After kswapd has reclaimed enough pages for free pages to be above the high watermark, it attempts to sleep for 100ms. One of three things happens next:

1. The number of free pages is still above the high watermark after 100ms. This causes kswapd to go back to sleep indefinitely without incrementing any stat.
2. The number of free pages drops below the high watermark after 100ms. This causes kswapd to keep running and increment the kswapd_high_wmark_hit_quickly stat.
3. The number of free pages drops below the low watermark before this 100ms sleep finishes. This causes kswapd to keep running and increment the kswapd_low_wmark_hit_quickly stat.

It's a little more complicated than that since there are other conditions besides watermarks that may cause kswapd to wake up or continue running, but that is the basic idea.

Sources:

• https://elixir.bootlin.com/linux/v5.0.3/source/mm/vmscan.c#L3805
• https://www.kernel.org/doc/gorman/html/understand/understand005.html#sec:%20Zone%20Watermarks