Hello, I'm posting second version of my IO-less balance_dirty_pages() patches. This is alternative approach to Fengguang's patches - much simpler I believe (only 300 lines added) - but obviously I does not provide so sophisticated control. Fengguang is currently running some tests on my patches so that we can compare the approaches. The basic idea (implemented in the third patch) is that processes throttled in balance_dirty_pages() wait for enough IO to complete. The waiting is implemented as follows: Whenever we decide to throttle a task in balance_dirty_pages(), task adds itself to a list of tasks that are throttled against that bdi and goes to sleep waiting to receive specified amount of page IO completions. Once in a while (currently HZ/10, in patch 5 the interval is autotuned based on observed IO rate), accumulated page IO completions are distributed equally among waiting tasks. This waiting scheme has been chosen so that waiting time in balance_dirty_pages() is proportional to number_waited_pages * number_of_waiters. In particular it does not depend on the total number of pages being waited for, thus providing possibly a fairer results. Since last version I've implemented cleanups as suggested by Peter Zilstra. The patches undergone more throughout testing. So far I've tested different filesystems (ext2, ext3, ext4, xfs, nfs), also a combination of a local filesystem and nfs. The load was either various number of dd threads or fio with several threads each dirtying pages at different speed. Results and test scripts can be found at http://beta.suse.com/private/jack/balance_dirty_pages-v2/ See README file for some explanation of test framework, tests, and graphs. Except for ext3 in data=ordered mode, where kjournald creates high fluctuations in waiting time of throttled processes (and also high latencies), the results look OK. Parallel dd threads are being throttled in the same way (in a 2s window threads spend the same time waiting) and also latencies of individual waits seem OK - except for ext3 they fit in 100 ms for local filesystems. They are in 200-500 ms range for NFS, which isn't that nice but to fix that we'd have to modify current ratelimiting scheme to take into account on which bdi a page is dirtied. Then we could ratelimit slower BDIs more often thus reducing latencies in individual waits... The results for different bandwidths fio load is interesting. There are 8 threads dirtying pages at 1,2,4,..,128 MB/s rate. Due to different task bdi dirty limits, what happens is that three most aggresive tasks get throttled so they end up at bandwidths 24, 26, and 30 MB/s and the lighter dirtiers run unthrottled. I'm planning to run some tests with multiple SATA drives to verify whether there aren't some unexpected fluctuations. But currently I have some trouble with the HW... As usual comments are welcome :). Honza -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
