Re: [RFC][PATCH 0/2] Tunable watermark
From: Ying Han
Date: Fri Jan 07 2011 - 17:35:37 EST
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
>> 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.)
>> 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
>> 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.
>> 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
> 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
I agree in general that adding the APIs to each wmarks sounds like a
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
> To unsubscribe, send a message with 'unsubscribe linux-mm' in
> the body to majordomo@xxxxxxxxxx For more info on Linux MM,
> see: http://www.linux-mm.org/ .
> Fight unfair telecom policy in Canada: sign http://dissolvethecrtc.ca/
> Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/