Re: [RFC PATCH v1 00/13] lru_lock scalability

[Steven Whitehouse]

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@xxxxxxxxxx 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.
> > >
> > > ÂÂÂÂÂÂÂÂ *ooooooooooo**ooooooooooo**ooooooooooo**oooo ...
> > > ÂÂÂÂÂÂÂÂ |ÂÂÂÂÂÂÂÂÂÂ ||ÂÂÂÂÂÂÂÂÂÂ ||ÂÂÂÂÂÂÂÂÂÂ ||
> > > ÂÂÂÂÂÂÂÂÂ \ batch 1 /Â \ batch 2 /Â \ 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
> > > ÂÂ 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.