92dc4ad83d
Entries in the dbuf cache contribute only the size of the dbuf data to the cache size. Attached "user" data is not counted. This can lead to the data currently "owned" by the cache consuming more memory accounting appears to show. In some cases (eg a metadnode data block with all child dnode_t slots allocated), the actual size can be as much as 3x as what the cache believes it to be. This is arguably correct behaviour, as the cache is only tracking the size of the dbuf data, not even the overhead of the dbuf_t. On the other hand, in the above case of dnodes, evicting cached metadnode dbufs is the only current way to reclaim the dnode objects, and can lead to the situation where the dbuf cache appears to be comfortably within its target memory window and yet is holding enormous amounts of slab memory that cannot be reclaimed. This commit adds a facility for a dbuf user to artificially inflate the apparent size of the dbuf for caching purposes. This at least allows for cache tuning to be adjusted to match something closer to the real memory overhead. metadnode dbufs carry a >1KiB allocation per dnode in their user data. This informs the dbuf cache machinery of that fact, allowing it to make better decisions when evicting dbufs. Sponsored-by: Klara, Inc. Sponsored-by: Wasabi Technology, Inc. Reviewed-by: Alexander Motin <mav@FreeBSD.org> Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov> Signed-off-by: Rob Norris <rob.norris@klarasystems.com> Closes #15511 |
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