Re: [PATCH-RFC] cfq: Disable low_latency by default for 2.6.32

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> On Thu, Nov 26, 2009 at 02:47:10PM +0100, Corrado Zoccolo wrote:
> > On Thu, Nov 26, 2009 at 1:19 PM, Mel Gorman <mel@csn.ul.ie> wrote:
> > > (cc'ing the people from the page allocator failure thread as this might be
> > > relevant to some of their problems)
> > >
> > > I know this is very last minute but I believe we should consider disabling
> > > the "low_latency" tunable for block devices by default for 2.6.32.  There was
> > > evidence that low_latency was a problem last week for page allocation failure
> > > reports but the reproduction-case was unusual and involved high-order atomic
> > > allocations in low-memory conditions. It took another few days to accurately
> > > show the problem for more normal workloads and it's a bit more wide-spread
> > > than just allocation failures.
> > >
> > > Basically, low_latency looks great as long as you have plenty of memory
> > > but in low memory situations, it appears to cause problems that manifest
> > > as reduced performance, desktop stalls and in some cases, page allocation
> > > failures. I think most kernel developers are not seeing the problem as they
> > > tend to test on beefier machines and without hitting swap or low-memory
> > > situations for the most part. When they are hitting low-memory situations,
> > > it tends to be for stress tests where stalls and low performance are expected.
> > 
> > The low latency tunable controls various policies inside cfq.
> > The one that could affect memory reclaim is:
> >         /*
> >          * Async queues must wait a bit before being allowed dispatch.
> >          * We also ramp up the dispatch depth gradually for async IO,
> >          * based on the last sync IO we serviced
> >          */
> >         if (!cfq_cfqq_sync(cfqq) && cfqd->cfq_latency) {
> >                 unsigned long last_sync = jiffies - cfqd->last_end_sync_rq;
> >                 unsigned int depth;
> > 
> >                 depth = last_sync / cfqd->cfq_slice[1];
> >                 if (!depth && !cfqq->dispatched)
> >                         depth = 1;
> >                 if (depth < max_dispatch)
> >                         max_dispatch = depth;
> >         }
> > 
> > here the async queues max depth is limited to 1 for up to 200 ms after
> > a sync I/O is completed.
> > Note: dirty page writeback goes through an async queue, so it is
> > penalized by this.
> > 
> > This can affect both low and high end hardware. My non-NCQ sata disk
> > can handle a depth of 2 when writing. NCQ sata disks can handle a
> > depth up to 31, so limiting depth to 1 can cause write performance
> > drop, and this in turn will slow down dirty page reclaim, and cause
> > allocation failures.
> > 
> > It would be good to re-test the OOM conditions with that code commented out.
> > 
> 
> All of it or just the cfq_latency part?
> 
> As it turns out the test machine does report for the disk NCQ (depth 31/32)
> and it's the same on the laptop so slowing down dirty page cleaning
> could be impacting reclaim.
> 
> > >
> > > To show the problem, I used an x86-64 machine booting booted with 512MB of
> > > memory. This is a small amount of RAM but the bug reports related to page
> > > allocation failures were on smallish machines and the disks in the system
> > > are not very high-performance.
> > >
> > > I used three tests. The first was sysbench on postgres running an IO-heavy
> > > test against a large database with 10,000,000 rows. The second was IOZone
> > > running most of the automatic tests with a record length of 4KB and the
> > > last was a simulated launching of gitk with a music player running in the
> > > background to act as a desktop-like scenario. The final test was similar
> > > to the test described here http://lwn.net/Articles/362184/ except that
> > > dm-crypt was not used as it has its own problems.
> > 
> > low_latency was tested on other scenarios:
> > http://lkml.indiana.edu/hypermail/linux/kernel/0910.0/01410.html
> > http://linux.derkeiler.com/Mailing-Lists/Kernel/2009-11/msg04855.html
> > where it improved actual and perceived performance, so disabling it
> > completely may not be good.
> > 
> 
> It may not indeed.
> 
> In case you mean a partial disabling of cfq_latency, I'm try the
> following patch. The intention is to disable the low_latency logic if
> kswapd is at work and presumably needs clean pages. Alternative
> suggestions welcome.

I like treat vmscan writeout as special. because
  - vmscan use various process context. but it doesn't write own process's page.
    IOW, it doesn't so match cfq's io fairness logic.
  - plus, the above mean vmscan writeout doesn't need good i/o latency.
  - vmscan maintain page granularity lru list. It mean vmscan makes awful
    seekful I/O. it assume block-layer buffered much i/o request.
  - plus, the above mena vmscan. writeout need good io throughput. otherwise
    system might cause hangup.

However, I don't think kswapd_awake is good choice. because
  - zone reclaim run before kswapd wakeup. iow, this patch doesn't solve hpc machine.
    btw, some Core i7 box (at least, Intel's reference box) also use zone reclaim.
  - On large (many memory node) machine, one of much kswapd always run.


Instead, PF_MEMALLOC is good idea?


Subject: [PATCH] cfq: Do not limit the async queue depth while memory reclaim

Not-Signed-off-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com> (I haven't test this)
---
 block/cfq-iosched.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)

diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c
index aa1e953..9546f64 100644
--- a/block/cfq-iosched.c
+++ b/block/cfq-iosched.c
@@ -1308,7 +1308,8 @@ static bool cfq_may_dispatch(struct cfq_data *cfqd, struct cfq_queue *cfqq)
 	 * We also ramp up the dispatch depth gradually for async IO,
 	 * based on the last sync IO we serviced
 	 */
-	if (!cfq_cfqq_sync(cfqq) && cfqd->cfq_latency) {
+	if (!cfq_cfqq_sync(cfqq) && cfqd->cfq_latency &&
+	    !(current->flags & PF_MEMALLOC)) {
 		unsigned long last_sync = jiffies - cfqd->last_end_sync_rq;
 		unsigned int depth;
 
-- 
1.6.5.2






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