Re: [PATCH] zsmalloc: merge size_class to reduce fragmentation
From: Andrew Morton <akpm@linux-foundation.org>
Date: 2014-09-23 22:25:57
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On Tue, 23 Sep 2014 17:30:11 +0900 Joonsoo Kim [off-list ref] 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.
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--- 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.
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/* 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.
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@@ -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? -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@kvack.org"> email@kvack.org </a>