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On fragmented pools with high-performance storage, the looping in metaslab_block_picker() can become the performance-limiting bottleneck. When looking for a larger block (e.g. a 128K block for the ZIL), we may search through many free segments (up to hundreds of thousands) to find one that is large enough to satisfy the allocation. This can take a long time (up to dozens of ms), and is done while holding the ms_lock, which other threads may spin waiting for. When this performance problem is encountered, profiling will show high CPU time in metaslab_block_picker, as well as in mutex_enter from various callers. The problem is very evident on a test system with a sync write workload with 8K writes to a recordsize=8k filesystem, with 4TB of SSD storage, 84% full and 88% fragmented. It has also been observed on production systems with 90TB of storage, 76% full and 87% fragmented. The fix is to change metaslab_df_alloc() to search only up to 16MB from the previous allocation (of this alignment). After that, we will pick a segment that is of the exact size requested (or larger). This reduces the number of iterations to a few hundred on fragmented pools (a ~100x improvement). Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Reviewed-by: Paul Dagnelie <pcd@delphix.com> Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com> Reviewed-by: George Wilson <george.wilson@delphix.com> Reviewed-by: Serapheim Dimitropoulos <serapheim@delphix.com> Signed-off-by: Matthew Ahrens <mahrens@delphix.com> External-issue: DLPX-62324 Closes #8877 |
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