Re: [PATCH v2 1/1] mm/ksm: improve deduplication of zero pages with colouring

[Andrew Morton]

On Thu, 19 Jan 2017 19:35:53 +0100 Claudio Imbrenda <imbrenda@xxxxxxxxxxxxxxxxxx> wrote:

> Some architectures have a set of zero pages (coloured zero pages)
> instead of only one zero page, in order to improve the cache
> performance. In those cases, the kernel samepage merger (KSM) would
> merge all the allocated pages that happen to be filled with zeroes to
> the same deduplicated page, thus losing all the advantages of coloured
> zero pages.
> 
> This behaviour is noticeable when a process accesses large arrays of
> allocated pages containing zeroes. A test I conducted on s390 shows
> that there is a speed penalty when KSM merges such pages, compared to
> not merging them or using actual zero pages from the start without
> breaking the COW.
> 
> This patch fixes this behaviour. When coloured zero pages are present,
> the checksum of a zero page is calculated during initialisation, and
> compared with the checksum of the current canditate during merging. In
> case of a match, the normal merging routine is used to merge the page
> with the correct coloured zero page, which ensures the candidate page
> is checked to be equal to the target zero page.
> 
> A sysfs entry is also added to toggle this behaviour, since it can
> potentially introduce performance regressions, especially on
> architectures without coloured zero pages. The default value is
> disabled, for backwards compatibility.
> 
> With this patch, the performance with KSM is the same as with non
> COW-broken actual zero pages, which is also the same as without KSM.
> 
> ...
>
> @@ -2233,6 +2267,28 @@ static ssize_t merge_across_nodes_store(struct kobject *kobj,
>  KSM_ATTR(merge_across_nodes);
>  #endif
>  
> +static ssize_t use_zero_pages_show(struct kobject *kobj,
> +				struct kobj_attribute *attr, char *buf)
> +{
> +	return sprintf(buf, "%u\n", ksm_use_zero_pages);
> +}
> +static ssize_t use_zero_pages_store(struct kobject *kobj,
> +				   struct kobj_attribute *attr,
> +				   const char *buf, size_t count)
> +{
> +	int err;
> +	bool value;
> +
> +	err = kstrtobool(buf, &value);
> +	if (err)
> +		return -EINVAL;
> +
> +	ksm_use_zero_pages = value;
> +
> +	return count;
> +}
> +KSM_ATTR(use_zero_pages);

Please send along an update for Documentation/vm/ksm.txt?  Be sure that
it fully explains "since it can potentially introduce performance
regressions", so our users are able to understand whether or not they
should use this.