On Wed, 11 Jan 2017 15:00:29 -0800 Andrew Morton <[email protected]> wrote:
> hm, bit_spin_lock() is a nasty thing. It is slow and it doesn't have > all the lockdep support. > > Would the world end if we added a spinlock to swap_cluster_info? FWIW, I asked the same question in December, this is what I got: jon > From: "Huang\, Ying" <[email protected]> > To: Jonathan Corbet <[email protected]> > Cc: Tim Chen <[email protected]>, Andrew Morton > <[email protected]>, "Huang\, Ying" <[email protected]>, > <[email protected]>, <[email protected]>, <[email protected]>, > <[email protected]>, <[email protected]>, Hugh Dickins > <[email protected]>, Shaohua Li <[email protected]>, Minchan Kim > <[email protected]>, Rik van Riel <[email protected]>, Andrea Arcangeli > <[email protected]>, "Kirill A . Shutemov" > <[email protected]>, Vladimir Davydov > <[email protected]>, Johannes Weiner <[email protected]>, Michal > Hocko <[email protected]>, "Hillf Danton" <[email protected]> > Subject: Re: [PATCH v2 2/8] mm/swap: Add cluster lock > Date: Tue, 25 Oct 2016 10:05:39 +0800 > > Hi, Jonathan, > > Thanks for review. > > Jonathan Corbet <[email protected]> writes: > > > On Thu, 20 Oct 2016 16:31:41 -0700 > > Tim Chen <[email protected]> wrote: > > > >> From: "Huang, Ying" <[email protected]> > >> > >> This patch is to reduce the lock contention of swap_info_struct->lock > >> via using a more fine grained lock in swap_cluster_info for some swap > >> operations. swap_info_struct->lock is heavily contended if multiple > >> processes reclaim pages simultaneously. Because there is only one lock > >> for each swap device. While in common configuration, there is only one > >> or several swap devices in the system. The lock protects almost all > >> swap related operations. > > > > So I'm looking at this a bit. Overall it seems like a good thing to do > > (from my limited understanding of this area) but I have a probably silly > > question... > > > >> struct swap_cluster_info { > >> - unsigned int data:24; > >> - unsigned int flags:8; > >> + unsigned long data; > >> }; > >> -#define CLUSTER_FLAG_FREE 1 /* This cluster is free */ > >> -#define CLUSTER_FLAG_NEXT_NULL 2 /* This cluster has no next cluster */ > >> +#define CLUSTER_COUNT_SHIFT 8 > >> +#define CLUSTER_FLAG_MASK ((1UL << CLUSTER_COUNT_SHIFT) - 1) > >> +#define CLUSTER_COUNT_MASK (~CLUSTER_FLAG_MASK) > >> +#define CLUSTER_FLAG_FREE 1 /* This cluster is free */ > >> +#define CLUSTER_FLAG_NEXT_NULL 2 /* This cluster has no next > >> cluster */ > >> +/* cluster lock, protect cluster_info contents and sis->swap_map */ > >> +#define CLUSTER_FLAG_LOCK_BIT 2 > >> +#define CLUSTER_FLAG_LOCK (1 << CLUSTER_FLAG_LOCK_BIT) > > > > Why the roll-your-own locking and data structures here? To my naive > > understanding, it seems like you could do something like: > > > > struct swap_cluster_info { > > spinlock_t lock; > > atomic_t count; > > unsigned int flags; > > }; > > > > Then you could use proper spinlock operations which, among other things, > > would make the realtime folks happier. That might well help with the > > cache-line sharing issues as well. Some of the count manipulations could > > perhaps be done without the lock entirely; similarly, atomic bitops might > > save you the locking for some of the flag tweaks - though I'd have to look > > more closely to be really sure of that. > > > > The cost, of course, is the growth of this structure, but you've already > > noted that the overhead isn't all that high; seems like it could be worth > > it. > > Yes. The data structure you proposed is much easier to be used than the > current one. The main concern is the RAM usage. The size of the data > structure you proposed is about 80 bytes, while that of the current one > is about 8 bytes. There will be one struct swap_cluster_info for every > 1MB swap space, so for 1TB swap space, the total size will be 80M > compared with 8M of current implementation. > > In the other hand, the return of the increased size is not overwhelming. > The bit spinlock on cluster will not be heavy contended because it is a > quite fine-grained lock. So the benefit will be little to use lockless > operations. I guess the realtime issue isn't serious given the lock is > not heavy contended and the operations protected by the lock is > light-weight too. > > Best Regards, > Huang, Ying > > > I assume that I'm missing something obvious here? > > > > Thanks, > > > > jon >
