There is a log:

Throttling is applied to page modifications [ percentOfPartTime=0.59, markDirty=7424 pages/sec, checkpointWrite=6268 pages/sec, estIdealMarkDirty=0 pages/sec, curDirty=0.00, maxDirty=0.24, avgParkTime=79770 ns, pages: (total=67085, evicted=0, written=40916, synced=0, cpBufUsed=3, cpBufTotal=518215)]

Although there are plenty of non-dirty pages, throttling is applied. This happens because speed-based throttling has 2 algorithms for protecting non-dirty pages from exhaustion:

1. A more complex one that computes max allowable dirty ratio and ideal marking speed and throttles when both dirty ratio and current marking speed surpass these values
2. A simpler one that throttles if the current marking speed is higher than the average checkpointing speed In the shown example the first algorithm does not throttle, but the second one does.

It looks like the throttling is enabled too early.

One way to solve this problem is to just disable the second algorithm as the first seems to be more adequate (but this needs careful consideration of all possible cases).

Another way is to consider averaged marking speed instead of (or in addition to) the current marking speed when deciding whether to throttle or not.