cap on writeback?

[Valdis.Kletnieks at vt.edu]

On Mon, 25 Mar 2013 17:23:40 -0700, Raymond Jennings said:

> Is there some sort of mechanism that throttles the size of the writeback pool?

There's a lot of tunables in /proc/sys/vm - everything from drop_caches
to swappiness to vfs_cache_pressure.  Note that they all interact in mystical
and hard-to-understand ways. ;)

> it's somewhat related to my brainfuck queue, since I would like to
> stress test it digesting a huge pile of outbound data and seeing if it
> can make writeback less seeky.

The biggest challenge here is that there's a bit of a layering violation
to be resolved - when the VM is choosing what pages get written out first,
it really has no clue where on disk the pages are going. Consider a 16M
file that's fragged into 16 1M extents - they'll almost certainly hit
the writeback queue in logical block order, not physical address order.
The only really good choices here are to either allow the writeback queue
to get deep enough that an elevator can do something useful (if you only
have 2-3 IOs queued, you can do less than if you have 20-30 of them you
can sort into some useful order), and filesystems that are less prone
to fragmentation issues

Just for the record, most of my high-performance stuff runs best with
the noop scheduler - when you're striping I/O across several hundred disks,
the last thing you want is some some single-minded disk scheduler re-arranging
the I/Os and creating latency issues for your striping.

Might want to think about why there's lots of man-hours spent doing
new filesystems and stuff like zcache and kernel shared memory, but the
only IO schedulers in tree are noop, deadline, and cfq :)
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