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Subject[PATCH 00/45] some writeback experiments
Hi all,

Here is a collection of writeback patches on

- larger writeback chunk sizes
- single per-bdi flush thread (killing the foreground throttling writeouts)
- lumpy pageout
- sync livelock prevention
- writeback scheduling
- random fixes

Sorry for posting a too big series - there are many direct or implicit
dependencies, and one patch lead to another before I can stop..

The lumpy pageout and nr_segments support is not complete and do not
cover all filesystems for now. It may be better to first convert some of
the ->writepages to the generic routines to avoid duplicate work.

I managed to address many issues in past week, however there are still known
problems. Hints from filesystem developers are highly appreciated. Thanks!

The estimated writeback bandwidth is about 1/2 the real throughput
for ext2/3/4 and btrfs; noticeable bigger than real throughput for NFS; and
cannot be estimated at all for XFS. Very interesting..

NFS writeback is very bumpy. The page numbers and network throughput "freeze"
together from time to time:

# vmmon -d 1 nr_writeback nr_dirty nr_unstable # (per 1-second samples)
nr_writeback nr_dirty nr_unstable
11227 41463 38044
11227 41463 38044
11227 41463 38044
11227 41463 38044
11045 53987 6490
11033 53120 8145
11195 52143 10886
11211 52144 10913
11211 52144 10913
11211 52144 10913

btrfs seems to maintain a private pool of writeback pages, which can go out of
control:

nr_writeback nr_dirty
261075 132
252891 195
244795 187
236851 187
228830 187
221040 218
212674 237
204981 237

XFS has very interesting "bumpy writeback" behavior: it tends to wait
collect enough pages and then write the whole world.

nr_writeback nr_dirty
80781 0
37117 37703
37117 43933
81044 6
81050 0
43943 10199
43930 36355
43930 36355
80293 0
80285 0
80285 0

Thanks,
Fengguang



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