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6988 spa_sync() spends half its time in dmu_objset_do_userquota_updates Using a benchmark which creates 2 million files in one TXG, I observe that the thread running spa_sync() is on CPU almost the entire time we are syncing, and therefore can be a performance bottleneck. About 50% of the time in spa_sync() is in dmu_objset_do_userquota_updates(). The problem is that dmu_objset_do_userquota_updates() calls zap_increment_int(DMU_USERUSED_OBJECT) once for every file that was modified (or created). In this benchmark, all the files are owned by the same user/group, so all 2 million calls to zap_increment_int() are modifying the same entry in the zap. The same issue exists for the DMU_GROUPUSED_OBJECT. We should keep an in-memory map from user to space delta while we are syncing, and when we finish, iterate over the in-memory map and modify the ZAP once per entry. This reduces the number of calls to zap_increment_int() from "number of objects modified" to "number of owners/groups of modified files". This reduced the time spent in spa_sync() in the file create benchmark by ~33%, from 11 seconds to 7 seconds. Closes #107 Reviewed by: George Wilson <george.wilson@delphix.com> Reviewed by: Steve Gonczi <steve.gonczi@delphix.com> Reviewed by: Ned Bass <bass6@llnl.gov> Reviewed by: Jinshan Xiong <jinshan.xiong@intel.com> Author: Matthew Ahrens <mahrens@delphix.com> openzfs/openzfs@5fc46359c5
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