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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