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

Corrado Zoccolo
2009-11-26T15:18:20Z

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.
>  */