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

Mike Galbraith
2009-11-26T13:09:03Z

On Thu, 2009-11-26 at 12:19 +0000, Mel Gorman 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.

Ouch. It was bad desktop stalls under heavy write that kicked the whole
thing off.

-Mike