Re: Delaying writes to disk when there's no need

[Nick Piggin <piggin@cyberone.com.au>]

Andrew Morton wrote:

>Nick Piggin <piggin@cyberone.com.au> wrote:
>  
>
>>it seems to me that
>>doing writeout whenever the disk would otherwise be idle
>>(and we have dirty memory to write out) would be a good
>>solution.
>>    
>>
>
>This is what the recently-removed BDI_read_active flag in backing_dev_info
>was supposed to be for.  I let it go because I don't think it's terribly
>important and it's time to stop fiddling with the vfs writeout code and it
>wasn't right anyway.
>
>Note that 2.5 starts pdflush writeout at 10% of memory dirty.  Or even lower
>if there is a lot of mapped memory around.  Whereas 2.4 will start background
>writeout at 30% or 40% dirty.  That's a fairly significant tuning change.
>
>The algorithm for utilisation of an idle disk should be, in
>balance_dirty_pages():
>
>	if (ps.nr_dirty + ps.nr_writeback < background_thresh) {
>		if (time_after(jiffies, bdi->last_read + HZ/100)) {
>			if (bdi->write_requests_in_flight < 2) {
>				struct writeback_control wbc = {
>					.bdi		= bdi,
>					.sync_mode	= WB_SYNC_NONE,
>					.nr_to_write	= write_chunk,
>				};
>
>				writeback_inodes(&wbc);
>			}
>		}
>		return;
>	}
>
>
>Or something like that.  It's pretty close.
>
Yeah something like that looks alright.

>
>It could have pretty bad failure modes.  Short-lived files in /tmp now
>perform writeout, which needs to be waited on when those files are removed.
>  
>
I didn't think of that.