Re: [RFC][PATCH 0/2] Tunable watermark

[Ying Han]

On Fri, Jan 7, 2011 at 2:23 PM, David Rientjes <rientjes@xxxxxxxxxx> wrote:
> On Fri, 7 Jan 2011, Satoru Moriya wrote:
>
>> This patchset introduces a new knob to control each watermark
>> separately.
>>
>> [Purpose]
>> To control the timing at which kswapd/direct reclaim starts(ends)
>> based on memory pressure and/or application characteristics
>> because direct reclaim makes a memory alloc/access latency worse.
>> (We'd like to avoid direct reclaim to keep latency low even if
>>  under the high memory pressure.)
>>
>> [Problem]
>> The thresholds kswapd/direct reclaim starts(ends) depend on
>> watermark[min,low,high] and currently all watermarks are set
>> based on min_free_kbytes. min_free_kbytes is the amount of
>> free memory that Linux VM should keep at least.
>>
>
> Not completely, it also depends on the amount of lowmem (because of the
> reserve setup next) and the amount of memory in each zone.
>
>> This means the difference between thresholds at which kswapd
>> starts and direct reclaim starts depends on the amount of free
>> memory.
>>
>> On the other hand, the amount of required memory depends on
>> applications. Therefore when it allocates/access memory more
>> than the difference between watemark[low] and watermark[min],
>> kernel sometimes runs direct reclaim before allocation and
>> it makes application latency bigger.
>>
>> [Solution]
>> To avoid the situation above, this patch set introduces new
>> tunables /proc/sys/vm/wmark_min_kbytes, wmark_low_kbytes and
>> wmark_high_kbytes. Each entry controls watermark[min],
>> watermark[low] and watermark[high] separately.
>> By using these parameters one can make the difference between
>> min and low bigger than the amount of memory which applications
>> require.
>>
>
> I really dislike this because it adds additional tunables that should
> already be handled correctly by the VM and it's very difficult for users
> to know what to tune these values to; these watermarks (with the exception
> of min) are supposed to be internal to the VM implementation.
>
> You didn't mention why it wouldn't be possible to modify
> setup_per_zone_wmarks() in some way for your configuration so this happens
> automatically.  If you can find a deterministic way to set these
> watermarks from userspace, you should be able to do it in the kernel as
> well based on the configuration.
>
> I think we should invest time in making sure the VM works for any type of
> workload thrown at it instead of relying on userspace making lots of
> adjustments.

I agree in general that adding the APIs to each wmarks sounds like a
over-kill, and
hard for user to configure most of the time.

On the other hand, having the low/high wmark consider more characters
other than the
size of the zone sounds useful. But I am not sure how to approach that
entirely in the
kernel if we like the reclaim behavior to be reflected from the
different workload.

--Ying

>
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