Andrew Morton wrote:
Shailabh Nagar <[email protected]> wrote:
delayacct-blkio-swapin.patch
Collect per-task block I/O delay statistics.
Unlike earlier iterations of the delay accounting
patches, now delays are only collected for the actual
I/O waits rather than try and cover the delays seen in
I/O submission paths.
Account separately for block I/O delays
incurred as a result of swapin page faults whose
frequency can be affected by the task/process' rss limit.
Hence swapin delays can act as feedback for rss limit changes
independent of I/O priority changes.
..
+#define PF_SWAPIN 0x02000000 /* I am doing a swap in */
Is there really no sane alternative to doing it this way?
I'm not sure there is. This goes back to what is really being measured
as "block I/O delay".
Earlier iterations of the code tried to measure total block I/O delay
including I/O submission
and the wait for completion:
http://www.uwsg.indiana.edu/hypermail/linux/kernel/0602.3/0905.html
In that design, the swapin delay was being measured as time spent in the
do_swap_page
function:
http://www.uwsg.indiana.edu/hypermail/linux/kernel/0602.3/0906.html
However, Arjan pointed out the difficulties in trying to account for all
I/O submission paths
so we reverted to measuring just the block I/O wait time within
io_schedule/io_schedule_timeout.
Since these functions will be called as a result of do_swap_page, the
only way to distinguish a
block I/O being done for a swapin vs. others was to mark the task struct
in some way and
task->flags seemed like a convenient way to do it.
We could easily introduce and use a separate flag variable in the delay
accounting structure itself if
using up another bit of task->flags is a concern ?
Or do you have doubts about the methodology of using a flag itself ?
Don't see any other lightweight
way of doing this.
--Shailabh
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