[LU-6775] Reduce memory footprint of ldlm_lock and ldlm_resource

• Type: Bug
• Resolution: Fixed
• Priority: Minor
• Fix Version/s: Lustre 2.8.0
• Severity: 3
• Assignee: Niu Yawei (Inactive)
• Reporter: Niu Yawei (Inactive)

Description

Following is quote from Andreas:

Another minor improvement that is possible is to shrink the size of ldlm_resource, by filling in holes in the structure. The current size is 320 bytes, with 12 bytes of holes (use "pahole lustre/ptlrpc/ptlrpc.ko" from the "dwarves" RPM to see this). Moving lr_refcount after lr_lock would remove 8 bytes of holes. Removing the unused lr_most_restr field would save another 16 bytes. That would allow 13 ldlm_resource structures to fit into a 4096-byte page instead of only 12. Making lr_itree optional (e.g. allocating it from its own slab in ldlm_extent_add_lock() only for extent locks, and making the pointer to it a union with lr_lvb_inode which is only used on the client) would reduce the size by 128 bytes, and allow 24 ldlm_resource structures per 4096-byte page, saving about 15GB on the MDS in the graph.

Shrinking ldlm_lock to fit more per slab would be a bit harder, but would be worthwhile because there are so many locks and would save many MB of space on the server. The ldlm_*callback pointers could be replaced by a single ldlm_callback_suite pointer, since these are always the same, saving 16 bytes. The l_exp_flock_hash and l_tree_node are used on different lock types, and are only used on the server. l_lru, l_conn_export, and l_lvb* only used on the client, while l_export is only used on the server. There are also a number of other fields in ldlm_lock already listed as being client- or server-only Putting these in a union could save a lot of space. This is relatively more complex to do and should be in a separate bug, and discussed with Alex.

Issue Links

• is related to: LU-6529 Server side lock limits to avoid unnecessary memory exhaustion