Background ========== Filesystems with ACL enabled generally need to read "system.posix_acl_access"/"system.posix_acl_default" xattr to get the access and default ACL. When filesystem is mounted with ACL enabled while files in the system have not set access/default ACL, the getattr() will run in vain while the round trip can decrease the performance in workload like "ls -lR".
For example, there's a 12% performance boost if erofs is mounted with "noacl" when running "ls -lR" workload on dataset [1] (given in [2]). We'd better offer a fastpath to boost the above workload, as well as other negative xattr lookup. Proposal ======== Introduce a per-inode bloom filter for xattrs to boost the negative xattr queries. As following shows, a 32-bit bloom filter is introduced for each inode, describing if a xattr with specific name exists on this inode. ``` struct erofs_xattr_ibody_header { - __le32 h_reserved; + __le32 h_map; /* bloom filter */ ... } ``` Following are some implementation details for bloom filter. 1. Reverse bit value -------------------- The bloom filter structure describes if a given data is inside the set. It will map the given data into several bits of the bloom filter map. The data must not exist inside the set if any mapped bit is 0, while the data may be not inside the set even if all mapped bits is 1. While in our use case, as erofs_xattr_ibody_header.h_map is previously a (all zero) reserved field, the bit value for the bloom filter has a reverse semantics in consideration for compatibility. That is, for a given data, the mapped bits will be cleared to 0. Thus for a previously built image without support for bloom filter, the bloom filter is all zero and when it's mounted by the new kernel with support for bloom filter, it can not determine if the queried xattr exists on the inode and thus will fallback to the original routine of iterating all on-disk xattrs to determine if the queried xattr exists. 2. The number of hash functions ------------------------------- The optimal value for the number of the hash functions (k) is (ln2 * m/n), where m stands the number of bits of the bloom filter map, while n stands the number of all candidates may be inside the set. In our use case, the number of common used xattr (n) is approximately 8, including system.[posix_acl_access|posix_acl_default], security.[capability|selinux] and security.[SMACK64|SMACK64TRANSMUTE|SMACK64EXEC|SMACK64MMAP]. Given the number of bits of the bloom filter (m) is 32, the optimal value for the number of the hash functions (k) is 2 (ln2 * m/n = 2.7). 3. The hash function -------------------- xxh32() is chosen as the hash function. Following are some tested statistics of several candidate hash functions. Listed are time (in millionsecond) consumed when these hash functions process input data in chunks of 24, 32, 64 and 4096 bytes. | 24 B | 32 B | 64 B | 4 KB --------+-------+-------+-------+------ jhash | 1325 | 2041 | 4016 | 2294 jhash2 | 1323 | 2035 | 4011 | 2310 crc16 | 7918 | 1056 | 2110 | 13784 crc32 | 1824 | 2436 | 4873 | 3107 crc32c | 2120 | 2708 | 5142 | 3109 xxhash | 1320 | 1967 | 2131 | 429 xxh32 | 1458 | 1358 | 1848 | 836 xxh64 | 1321 | 2081 | 2128 | 429 3.1. multiple hash functions with various seeds ----------------------------------------------- As previously described, the given data will be mapped into several bits of the bloom filter map with hash functions. There could be several hash functions (k), with each hash function mapping the given data into one bit of the bloom filter map. Thus given the number of hash functions (k), each xattr name will be mapped into k bits of the bloom filter map. Here in our use case, k hash functions are all xxh32() but with different seeds. As following shows, the seed is (index + i), where i stands the index of the current hash function among all hash functions. In this way, each hash function is distinguishable with others. ``` for (i = 0; i < k; i++) bit = xxh32(name, strlen(name), index + i); ``` 3.2. input of hash function ------------------------- As previously described, each hash function will map the given data into one bit of the bloom filter map. In our use case, xattr name serves as the key of hash function. When .getxattr() gets called, only index (e.g. EROFS_XATTR_INDEX_USER) and the remaining name apart from the prefix are handy. To avoid constructing the full xattr name, the above index and name are fed into the hash function directly in the following way: ``` bit = xxh32(name, strlen(name), index + i); ``` where index serves as part of seed, so that it gets involved in the calculation for the hash. An alternative way is to calculate the hash from the full xattr name by feeding the prefix string and the remaining name string separately in the following way: ``` xxh32_reset() xxh32_update(prefix string, ...) xxh32_update(remaining name, ...) xxh32_digest() ``` But I doubt if it really deserves to call multiple APIs instead of one single xxh32(). Also be noted that for xattrs with long xattr name prefixes, the above "name" is the xattr name after stripping the corresponding short predefined xattr name prefix rather than the long xattr name prefix, as only the former is handy in the kernel routine. 3.3. discussions ---------------- I think a wider discussion on the implementation details is needed, including the number of the hash functions, and all other implementation details mentioned above, as they are also part of the on-disk format. Performance Improvement ======================= The performance statistics are tested with 'ls -lR' workload upon the dataset [1]. | uncached(ms) | cached(ms) ------------------------+---------------+---------- erofs.share | 468 | 264 erofs.share.bloom | 370 | 254 erofs.share.noacl | 412 | 216 erofs.share.noacl.bloom | 318 | 206 The tested erofs image is built in shared xattr layout. It indicates ~20% performance improvement with bloom filter in uncached scenarios. [1] https://my.owndrive.com/index.php/s/irHJXRpZHtT3a5i [2] https://lore.kernel.org/all/071074ad149b189661681aada453995741f75039.ca...@redhat.com/ Jingbo Xu (2): erofs: update on-disk format for xattr bloom filter erofs: optimize getxattr with bloom filter fs/erofs/erofs_fs.h | 8 +++++++- fs/erofs/internal.h | 2 ++ fs/erofs/xattr.c | 16 +++++++++++++++- 3 files changed, 24 insertions(+), 2 deletions(-) -- 2.19.1.6.gb485710b