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By aligning n_buckets to power of 2 we can turn the "bucket = hashval % n_buckets" into a less expensive bucket = hashval & (n_buckets - 1). This removes the DIV instruction as shown below. Before: xor %edx,%edx divl 0x8(%rbx) mov %edx,%eax add $0x1,%rax shl $0x4,%rax add %rbx,%rax After: lea -0x1(%rdi),%edx and %edx,%eax add $0x1,%rax shl $0x4,%rax add %rbx,%rax With a microbenchmark, measuring the time to locate the bucket (i.e. time_to_calculate_hashval + time_to_calculate_bucket_position) we have the results below (time in clock cycles): keylen before after 2-10 79.0 61.9 (-21.65%) 11-17 81.0 64.4 (-20.48%) 18-25 90.0 73.2 (-18.69%) 26-32 104.7 87.0 (-16.82%) 33-40 108.4 89.6 (-17.37%) 41-48 111.2 91.9 (-17.38%) 49-55 120.1 102.1 (-15.04%) 56-63 134.4 115.7 (-13.91%) As expected the gain is constant, regardless of the key length. The time to clculate the hashval varies with the key length, which explains the bigger gains for short keys. |
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.. | ||
docs | ||
.gitignore | ||
COPYING | ||
libkmod-array.c | ||
libkmod-array.h | ||
libkmod-config.c | ||
libkmod-elf.c | ||
libkmod-file.c | ||
libkmod-hash.c | ||
libkmod-hash.h | ||
libkmod-index.c | ||
libkmod-index.h | ||
libkmod-internal.h | ||
libkmod-list.c | ||
libkmod-module.c | ||
libkmod-signature.c | ||
libkmod-util.c | ||
libkmod-util.h | ||
libkmod.c | ||
libkmod.h | ||
libkmod.pc.in | ||
libkmod.sym | ||
macro.h | ||
missing.h | ||
README |
libkmod - linux kernel module handling library ABSTRACT ======== libkmod was created to allow programs to easily insert, remove and list modules, also checking its properties, dependencies and aliases. there is no shared/global context information and it can be used by multiple sites on a single program, also being able to be used from threads, although it's not thread safe (you must lock explicitly). OVERVIEW ======== Every user should create and manage it's own library context with: struct kmod_ctx *ctx = kmod_new(kernel_dirname); kmod_unref(ctx); Modules can be created by various means: struct kmod_module *mod; int err; err = kmod_module_new_from_path(ctx, path, &mod); if (err < 0) { /* code */ } else { /* code */ kmod_module_unref(mod); } err = kmod_module_new_from_name(ctx, name, &mod); if (err < 0) { /* code */ } else { /* code */ kmod_module_unref(mod); } Or could be resolved from a known alias to a list of alternatives: struct kmod_list *list, *itr; int err; err = kmod_module_new_from_lookup(ctx, alias, &list); if (err < 0) { /* code */ } else { kmod_list_foreach(itr, list) { struct kmod_module *mod = kmod_module_get_module(itr); /* code */ } }