Re: [RFC][PATCH 0/3] VM throttling: avoid blocking occasionalwriters

From: KAMEZAWA Hiroyuki
Date: Fri Feb 23 2007 - 23:49:00 EST

On Fri, 23 Feb 2007 21:03:37 +0900
Tomoki Sekiyama <tomoki.sekiyama.qu@xxxxxxxxxxx> wrote:

> Hi,
> I have observed a problem that write(2) can be blocked for a long time
> if a system has several disks and is under heavy I/O pressure. This
> patchset is to avoid the problem.
> Example of the probrem:
> There are two processes on a system which has two disks. Process-A
> writes heavily to disk-a, and process-B writes small data (e.g. log
> files) to disk-b occasionally. A portion of system memory, which is
> depends on vm.dirty_ratio (typically 40%), is filled up with Dirty
> and Writeback pages of disk-a.
> In this situation, write(2) of process-B could be blocked for a very
> long time (more then 60 seconds), although the load of disk-b is quite
> low. In particular, the system would become quite slow, if disk-a is
> slow (e.g. backup to an USB disk).
> This seems to be the same problem as discussed in LKML:
> and
Interesting, but how about adjust this parameter like below instead of
adding new control knob ?(this kind of knob is not easy to use.)

struct writeback_control wbc = {
.bdi = bdi,
.sync_mode = WB_SYNC_NONE,
.older_than_this = NULL,
.nr_to_write = 0,
.range_cyclic = 1,
if (nr_reclaimable) {
/* Just do what I can do */
dirty_pages_on_device = count_dirty_pages_on_device_limited(bdi, writechunk);
wbc.nr_to_write = dirty_pages_on_device.


count_dirty_pages_on_device_limited(bdi, writechunk) above returns
dirty pages on bdi. if # of dirty_pages on bdi is larger than writechunk,
just returns writechunk.


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