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

    >
    > 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.

    Thanks,
    Corrado
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