Re: [PATCH] mm:Add watermark slope for high mark
From: Michal Hocko <mhocko@kernel.org>
Date: 2017-11-24 13:27:24
Also in:
linux-fsdevel, linux-mm, lkml
On Fri 24-11-17 14:12:56, peter enderborg wrote:
On 11/24/2017 11:14 AM, Michal Hocko wrote:quoted
On Fri 24-11-17 11:07:07, Peter Enderborg wrote:quoted
When tuning the watermark_scale_factor to reduce stalls and compactions the high mark is also changed, it changed a bit too much. So this patch introduces a slope that can reduce this overhead a bit, or increase it if needed.This doesn't explain what is the problem, why it is a problem and why we need yet another tuning to address it. Users shouldn't really care about internal stuff like watermark tuning for each watermark independently. This looks like a gross hack. Please start over with the problem description and then we can move on to an approapriate fix. Piling up tuning knobs to workaround problems is simply not acceptable.In the original patch - https://lkml.org/lkml/2016/2/18/498 - had a discussion about small systems with 8GB RAM. In the handheld world, that's a lot of RAM. However, the magic number 2 used in the present algorithm is out of the blue. Compaction problems are the same for both small and big. So small devices also need to increase watermark to get compaction to work and reduce direct reclaims. Changing the low watermark makes direct reclaim rate drop a lot. But it will cause kswap to work more, and that has a negative impact. Lowering the gap will smooth out the kswap workload to suite embedded devices a lot better. This can be addressed by reducing the high watermark using the slope patch herein. Im sort of understand your opinion on user knobs, but hard-coded magic numbers are even worse.
How can a poor user know how to tune it when _we_ cannot do a qualified guess and we do know all the implementation details. Really, describe problems you are seeing with the current code and we can talk about a proper fix or a heuristic when the fix is hard/unfeasible. -- Michal Hocko SUSE Labs -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>