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68cbd56e18
Only attempt to backfill lower metadnode object numbers if at least 4096 objects have been freed since the last rescan, and at most once per transaction group. This avoids a pathology in dmu_object_alloc() that caused O(N^2) behavior for create-heavy workloads and substantially improves object creation rates. As summarized by @mahrens in #4636: "Normally, the object allocator simply checks to see if the next object is available. The slow calls happened when dmu_object_alloc() checks to see if it can backfill lower object numbers. This happens every time we move on to a new L1 indirect block (i.e. every 32 * 128 = 4096 objects). When re-checking lower object numbers, we use the on-disk fill count (blkptr_t:blk_fill) to quickly skip over indirect blocks that don’t have enough free dnodes (defined as an L2 with at least 393,216 of 524,288 dnodes free). Therefore, we may find that a block of dnodes has a low (or zero) fill count, and yet we can’t allocate any of its dnodes, because they've been allocated in memory but not yet written to disk. In this case we have to hold each of the dnodes and then notice that it has been allocated in memory. The end result is that allocating N objects in the same TXG can require CPU usage proportional to N^2." Add a tunable dmu_rescan_dnode_threshold to define the number of objects that must be freed before a rescan is performed. Don't bother to export this as a module option because testing doesn't show a compelling reason to change it. The vast majority of the performance gain comes from limit the rescan to at most once per TXG. Signed-off-by: Ned Bass <bass6@llnl.gov> Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov> |
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