Messages in this thread | | | Subject | Re: [RFC PATCH v1 00/13] lru_lock scalability | From | Steven Whitehouse <> | Date | Fri, 2 Feb 2018 10:50:37 +0000 |
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Hi,
On 02/02/18 04:18, Daniel Jordan wrote: > > > On 02/01/2018 10:54 AM, Steven Whitehouse wrote: >> Hi, >> >> >> On 31/01/18 23:04, daniel.m.jordan@oracle.com wrote: >>> lru_lock, a per-node* spinlock that protects an LRU list, is one of the >>> hottest locks in the kernel. On some workloads on large machines, it >>> shows up at the top of lock_stat. >>> >>> One way to improve lru_lock scalability is to introduce an array of >>> locks, >>> with each lock protecting certain batches of LRU pages. >>> >>> *ooooooooooo**ooooooooooo**ooooooooooo**oooo ... >>> | || || || >>> \ batch 1 / \ batch 2 / \ batch 3 / >>> >>> In this ASCII depiction of an LRU, a page is represented with either >>> '*' >>> or 'o'. An asterisk indicates a sentinel page, which is a page at the >>> edge of a batch. An 'o' indicates a non-sentinel page. >>> >>> To remove a non-sentinel LRU page, only one lock from the array is >>> required. This allows multiple threads to remove pages from different >>> batches simultaneously. A sentinel page requires lru_lock in >>> addition to >>> a lock from the array. >>> >>> Full performance numbers appear in the last patch in this series, >>> but this >>> prototype allows a microbenchmark to do up to 28% more page faults per >>> second with 16 or more concurrent processes. >>> >>> This work was developed in collaboration with Steve Sistare. >>> >>> Note: This is an early prototype. I'm submitting it now to support my >>> request to attend LSF/MM, as well as get early feedback on the >>> idea. Any >>> comments appreciated. >>> >>> >>> * lru_lock is actually per-memcg, but without memcg's in the picture it >>> becomes per-node. >> GFS2 has an lru list for glocks, which can be contended under certain >> workloads. Work is still ongoing to figure out exactly why, but this >> looks like it might be a good approach to that issue too. The main >> purpose of GFS2's lru list is to allow shrinking of the glocks under >> memory pressure via the gfs2_scan_glock_lru() function, and it looks >> like this type of approach could be used there to improve the >> scalability, > > Glad to hear that this could help in gfs2 as well. > > Hopefully struct gfs2_glock is less space constrained than struct page > for storing the few bits of metadata that this approach requires. > > Daniel > We obviously want to keep gfs2_glock small, however within reason then yet we can add some additional fields as required. The use case is pretty much a standard LRU list, so items are added and removed, mostly at the active end of the list, and the inactive end of the list is scanned periodically by gfs2_scan_glock_lru()
Steve.
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