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.
*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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