xfs: Lower CIL flush limit for large logs
The current CIL size aggregation limit is 1/8th the log size. This means for large logs we might be aggregating at least 250MB of dirty objects in memory before the CIL is flushed to the journal. With CIL shadow buffers sitting around, this means the CIL is often consuming >500MB of temporary memory that is all allocated under GFP_NOFS conditions. Flushing the CIL can take some time to do if there is other IO ongoing, and can introduce substantial log force latency by itself. It also pins the memory until the objects are in the AIL and can be written back and reclaimed by shrinkers. Hence this threshold also tends to determine the minimum amount of memory XFS can operate in under heavy modification without triggering the OOM killer. Modify the CIL space limit to prevent such huge amounts of pinned metadata from aggregating. We can have 2MB of log IO in flight at once, so limit aggregation to 16x this size. This threshold was chosen as it little impact on performance (on 16-way fsmark) or log traffic but pins a lot less memory on large logs especially under heavy memory pressure. An aggregation limit of 8x had 5-10% performance degradation and a 50% increase in log throughput for the same workload, so clearly that was too small for highly concurrent workloads on large logs. This was found via trace analysis of AIL behaviour. e.g. insertion from a single CIL flush: xfs_ail_insert: old lsn 0/0 new lsn 1/3033090 type XFS_LI_INODE flags IN_AIL $ grep xfs_ail_insert /mnt/scratch/s.t |grep "new lsn 1/3033090" |wc -l 1721823 $ So there were 1.7 million objects inserted into the AIL from this CIL checkpoint, the first at 2323.392108, the last at 2325.667566 which was the end of the trace (i.e. it hadn't finished). Clearly a major problem. Signed-off-by:Dave Chinner <dchinner@redhat.com> Reviewed-by:
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