Add docs for server overload

Author: Alex Katsman

docs/en/operations/settings/server-overload.md

@@ -0,0 +1,34 @@
+---
+description: 'Controlling behavior on server CPU overload.'
+slug: /operations/settings/server-overload
+title: 'Server overload'
+---
+
+# Server overload
+
+## Overview {#overview}
+
+Sometimes server can become overloaded due to different reasons. In order to determine the current CPU overload,
+ClickHouse server calculates the ratio of CPU wait time (`OSCPUWaitMicroseconds` metric) to busy time
+(`OSCPUVirtualTimeMicroseconds` metric). When the server is overloaded above certain ratio,
+it makes sense to discard some queries or even drop connection requests to not increase the load even more.
+
+There's a server setting `os_cpu_busy_time_threshold` which controls the minimum busy time to consider CPU
+doing some useful work. If the current value of `OSCPUVirtualTimeMicroseconds` metric is below this value,
+CPU overload is assumed to be 0.
+
+## Rejecting queries
+
+The behavior of rejecting queries is controlled by query-level settings `min_os_cpu_wait_time_ratio_to_throw` and
+`max_os_cpu_wait_time_ratio_to_throw`. If those settings are set and `min_os_cpu_wait_time_ratio_to_throw` is less
+than `max_os_cpu_wait_time_ratio_to_throw`, then the query is rejected and `SERVER_OVERLOADED` error is thrown
+with some probability is the overload ratio is at least `min_os_cpu_wait_time_ratio_to_throw`. The probability
+is determined as a linear interpolation between min and max ratios. For example, if `min_os_cpu_wait_time_ratio_to_throw = 2`,
+`max_os_cpu_wait_time_ratio_to_throw = 6`, and `cpu_overload = 4`, then the query will be rejected with a probability of `0.5`.
+
+## Dropping connections
+
+Dropping connections is controlled by server-level settings `min_os_cpu_wait_time_ratio_to_drop_connection` and
+`max_os_cpu_wait_time_ratio_to_drop_connection`. Those settings can be changed without server restart. The idea behind
+those settings is similar to the one with rejecting queries. The only difference in this case is if the server is overloaded,
+the connection attempt will be rejected from the server side.