Re: [PATCH] zsmalloc: merge size_class to reduce fragmentation

[Andrew Morton]

On Tue, 23 Sep 2014 17:30:11 +0900 Joonsoo Kim <iamjoonsoo.kim@xxxxxxx> wrote:

> zsmalloc has many size_classes to reduce fragmentation and they are
> in 16 bytes unit, for example, 16, 32, 48, etc., if PAGE_SIZE is 4096.
> And, zsmalloc has constraint that each zspage has 4 pages at maximum.
> 
> In this situation, we can see interesting aspect.
> Let's think about size_class for 1488, 1472, ..., 1376.
> To prevent external fragmentation, they uses 4 pages per zspage and
> so all they can contain 11 objects at maximum.
> 
> 16384 (4096 * 4) = 1488 * 11 + remains
> 16384 (4096 * 4) = 1472 * 11 + remains
> 16384 (4096 * 4) = ...
> 16384 (4096 * 4) = 1376 * 11 + remains
> 
> It means that they have same chracteristics and classification between
> them isn't needed. If we use one size_class for them, we can reduce
> fragementation and save some memory. Below is result of my simple test.
> 
> TEST ENV: EXT4 on zram, mount with discard option
> WORKLOAD: untar kernel source code, remove directory in descending order
> in size. (drivers arch fs sound include net Documentation firmware
> kernel tools)
> 
> Each line represents orig_data_size, compr_data_size, mem_used_total,
> fragmentation overhead (mem_used - compr_data_size) and overhead ratio
> (overhead to compr_data_size), respectively, after untar and remove
> operation is executed.
> 
> * untar-nomerge.out
> 
> orig_size compr_size used_size overhead overhead_ratio
> 525.88MB 199.16MB 210.23MB  11.08MB 5.56%
> 288.32MB  97.43MB 105.63MB   8.20MB 8.41%
> 177.32MB  61.12MB  69.40MB   8.28MB 13.55%
> 146.47MB  47.32MB  56.10MB   8.78MB 18.55%
> 124.16MB  38.85MB  48.41MB   9.55MB 24.58%
> 103.93MB  31.68MB  40.93MB   9.25MB 29.21%
>  84.34MB  22.86MB  32.72MB   9.86MB 43.13%
>  66.87MB  14.83MB  23.83MB   9.00MB 60.70%
>  60.67MB  11.11MB  18.60MB   7.49MB 67.48%
>  55.86MB   8.83MB  16.61MB   7.77MB 88.03%
>  53.32MB   8.01MB  15.32MB   7.31MB 91.24%
> 
> * untar-merge.out
> 
> orig_size compr_size used_size overhead overhead_ratio
> 526.23MB 199.18MB 209.81MB  10.64MB 5.34%
> 288.68MB  97.45MB 104.08MB   6.63MB 6.80%
> 177.68MB  61.14MB  66.93MB   5.79MB 9.47%
> 146.83MB  47.34MB  52.79MB   5.45MB 11.51%
> 124.52MB  38.87MB  44.30MB   5.43MB 13.96%
> 104.29MB  31.70MB  36.83MB   5.13MB 16.19%
>  84.70MB  22.88MB  27.92MB   5.04MB 22.04%
>  67.11MB  14.83MB  19.26MB   4.43MB 29.86%
>  60.82MB  11.10MB  14.90MB   3.79MB 34.17%
>  55.90MB   8.82MB  12.61MB   3.79MB 42.97%
>  53.32MB   8.01MB  11.73MB   3.73MB 46.53%
> 
> As you can see above result, merged one has better utilization (overhead
> ratio, 5th column) and uses less memory (mem_used_total, 3rd column).
> 

The above is great, but it provided no description of the implementation,
and there are no code comments describing what's going on either.

> --- a/mm/zsmalloc.c
> +++ b/mm/zsmalloc.c
> @@ -193,6 +193,7 @@ struct size_class {
>  	 */
>  	int size;
>  	unsigned int index;
> +	unsigned int nr_obj;

Documenting the data structures is critical.  If the roles and
relationships and interactions between the data structures are
skilfully described, the implementation tends to become relatively
obvious.

>  	/* Number of PAGE_SIZE sized pages to combine to form a 'zspage' */
>  	int pages_per_zspage;
> @@ -214,7 +215,8 @@ struct link_free {
>  };
>  
>  struct zs_pool {
> -	struct size_class size_class[ZS_SIZE_CLASSES];
> +	struct size_class *size_class[ZS_SIZE_CLASSES];
> +	struct size_class __size_class[ZS_SIZE_CLASSES];

Are these the best possible names?

I assume the entries in size_class[] point into entries in
__size_class[].  Some description of how (and why!) this is arranged
would go a long way.

> @@ -949,20 +961,28 @@ struct zs_pool *zs_create_pool(gfp_t flags)
>  	if (!pool)
>  		return NULL;
>  
> -	for (i = 0; i < ZS_SIZE_CLASSES; i++) {
> +	for (i = ZS_SIZE_CLASSES - 1; i >= 0; i--) {
>  		int size;
>  		struct size_class *class;
> +		struct size_class *prev_class;
>  
>  		size = ZS_MIN_ALLOC_SIZE + i * ZS_SIZE_CLASS_DELTA;
>  		if (size > ZS_MAX_ALLOC_SIZE)
>  			size = ZS_MAX_ALLOC_SIZE;
>  
> -		class = &pool->size_class[i];
> +		class = &pool->__size_class[i];
>  		class->size = size;
>  		class->index = i;
>  		spin_lock_init(&class->lock);
>  		class->pages_per_zspage = get_pages_per_zspage(size);
> +		class->nr_obj = class->pages_per_zspage * PAGE_SIZE / size;
>  
> +		pool->size_class[i] = class;
> +		if (i < ZS_SIZE_CLASSES - 1) {
> +			prev_class = pool->size_class[i + 1];
> +			if (is_same_density(prev_class, class))
> +				pool->size_class[i] = prev_class;
> +		}
>  	}

This is the key part and is a great place to explain your design to your
readers.


Please, let's do better than this?


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