Re: Why PAGEOUT_IO_SYNC stalls for a long time

[Mel Gorman <mel@csn.ul.ie>]

On Fri, Jul 30, 2010 at 01:54:53PM +0900, KOSAKI Motohiro wrote:
> > > (1) and (8) might be solved
> > > by sleeping awhile, but it's unrelated on io-congestion. but might not be. It only works
> > > by lucky. So I don't like to depned on luck. 
> > 
> > In this case, waiting a while really in the right thing to do. It stalls
> > the caller, but it's a high-order allocation. The alternative is for it
> > to keep scanning which when under memory pressure could result in far
> > too many pages being evicted. How long to wait is a tricky one to answer
> > but I would recommend making this a low priority.
> 
> For case (1), just lock_page() instead trylock is brilliant way than random sleep. 
> Is there any good reason to give up synchrounous lumpy reclaim when trylock_page() failed?
> IOW, briefly lock_page() and wait_on_page_writeback() have the same latency. why should
> we only avoid former?
> 

No reason. Using lock_page() in the synchronous case would be a sensible
choice. As you are realising, there are a number of warts around lumpy
reclaim that are long overdue for a good look :/

> side note: page lock contention is very common case.
> 
> For case (8), I don't think sleeping is right way. get_page() is used in really various place of
> our kernel. so we can't assume it's only temporary reference count increasing.

In what case is a munlocked pages reference count permanently increased and
why is this not a memory leak?

> In the other
> hand, this contention is not so common because shrink_page_list() is excluded from IO
> activity by page-lock and wait_on_page_writeback(). so I think giving up this case don't
> makes too many pages eviction.
> If you disagree, can you please explain your expected bad scinario?
> 

Right now, I can't think of a problem with calling lock_page instead of
trylock for synchronous lumpy reclaim.

> > > > > 3. pageout() is intended anynchronous api. but doesn't works so.
> > > > > 
> > > > > pageout() call ->writepage with wbc->nonblocking=1. because if the system have
> > > > > default vm.dirty_ratio (i.e. 20), we have 80% clean memory. so, getting stuck
> > > > > on one page is stupid, we should scan much pages as soon as possible.
> > > > > 
> > > > > HOWEVER, block layer ignore this argument. if slow usb memory device connect
> > > > > to the system, ->writepage() will sleep long time. because submit_bio() call
> > > > > get_request_wait() unconditionally and it doesn't have any PF_MEMALLOC task
> > > > > bonus.
> > > > 
> > > > Is this not a problem in the writeback layer rather than pageout()
> > > > specifically?
> > > 
> > > Well, outside pageout(), probably only XFS makes PF_MEMALLOC + writeout. 
> > > because PF_MEMALLOC is enabled only very limited situation. but I don't know
> > > XFS detail at all. I can't tell this area...
> > > 
> > 
> > All direct reclaimers have PF_MEMALLOC set so it's not that limited a
> > situation. See here
> 
> Yes, all direct reclaimers have PF_MEMALLOC. but usually all direct reclaimers don't call
> any IO related function except pageout(). As far as I know, current shrink_icache() and 
> shrink_dcache() doesn't make IO. Am I missing something?
> 

Not that I'm aware of but it's not something I would know offhand. Will
go digging.

-- 
Mel Gorman
Part-time Phd Student                          Linux Technology Center
University of Limerick                         IBM Dublin Software Lab