Messages in this thread | | | Date | Thu, 26 Nov 2009 16:18:18 +0100 | Subject | Re: [PATCH-RFC] cfq: Disable low_latency by default for 2.6.32 | From | Corrado Zoccolo <> |
| |
On Thu, Nov 26, 2009 at 3:17 PM, Mel Gorman <mel@csn.ul.ie> wrote: > 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? The whole if, that is enabled only with cfq_latency.
> > 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. Yes, I think so.
> >> > >> > 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. Yes, I meant exactly to disable that part, and doing it when kswapd is active is probably a good choice. I have a different idea for 2.6.33, though. If you have a reliable reproducer of the issue, can you test it on git://git.kernel.dk/linux-2.6-block.git branch for-2.6.33? It may already be unaffected, since we had various performance improvements there, but I think a better way to boost writeback is possible.
Thanks, Corrado
> > ====== > cfq: Do not limit the async queue depth while kswapd is awake > > diff --git a/block/cfq-iosched.c b/block/cfq-iosched.c > index aa1e953..dcab74e 100644 > --- a/block/cfq-iosched.c > +++ b/block/cfq-iosched.c > @@ -1308,7 +1308,7 @@ 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 && !kswapd_awake()) { > unsigned long last_sync = jiffies - cfqd->last_end_sync_rq; > unsigned int depth; > > diff --git a/include/linux/mmzone.h b/include/linux/mmzone.h > index 6f75617..b593aff 100644 > --- a/include/linux/mmzone.h > +++ b/include/linux/mmzone.h > @@ -655,6 +655,7 @@ typedef struct pglist_data { > void get_zone_counts(unsigned long *active, unsigned long *inactive, > unsigned long *free); > void build_all_zonelists(void); > +int kswapd_awake(void); > void wakeup_kswapd(struct zone *zone, int order); > int zone_watermark_ok(struct zone *z, int order, unsigned long mark, > int classzone_idx, int alloc_flags); > diff --git a/mm/vmscan.c b/mm/vmscan.c > index 777af57..75cdd9a 100644 > --- a/mm/vmscan.c > +++ b/mm/vmscan.c > @@ -2201,6 +2201,15 @@ static int kswapd(void *p) > return 0; > } > > +int kswapd_awake(void) > +{ > + pg_data_t *pgdat; > + for_each_online_pgdat(pgdat) > + if (!waitqueue_active(&pgdat->kswapd_wait)) > + return 1; > + return 0; > +} > + > /* > * A zone is low on free memory, so wake its kswapd task to service it. > */ > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |