Yeah, I think we default to a 10% "dirty background memory" (and
allows up to 20% dirty), so on your 16GB machine, we allow up to 1.6GB
of dirty memory for writeout before we even start writing, and twice
that before we start *waiting* for it.
On 32-bit x86, we only count the memory in the low 1GB (really
actually up to about 890MB), so "10% dirty" really means just about
90MB of buffering (and a "hard limit" of ~180MB of dirty).
=> On 32-bit system, the page cache also can use the high memory, so the size of 10% "dirty background memory" maybe 1.6GB for this case.