From: Chris Li <chrisl@kernel.org>
To: "Huang, Ying" <ying.huang@intel.com>
Cc: Yosry Ahmed <yosryahmed@google.com>,
lsf-pc@lists.linux-foundation.org,
Johannes Weiner <hannes@cmpxchg.org>,
Linux-MM <linux-mm@kvack.org>, Michal Hocko <mhocko@kernel.org>,
Shakeel Butt <shakeelb@google.com>,
David Rientjes <rientjes@google.com>,
Hugh Dickins <hughd@google.com>,
Seth Jennings <sjenning@redhat.com>,
Dan Streetman <ddstreet@ieee.org>,
Vitaly Wool <vitaly.wool@konsulko.com>,
Yang Shi <shy828301@gmail.com>, Peter Xu <peterx@redhat.com>,
Minchan Kim <minchan@kernel.org>,
Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [LSF/MM/BPF TOPIC] Swap Abstraction / Native Zswap
Date: Thu, 9 Mar 2023 11:58:53 -0800 [thread overview]
Message-ID: <ZAo6fc0wRtLJL+1T@google.com> (raw)
In-Reply-To: <87356e850j.fsf@yhuang6-desk2.ccr.corp.intel.com>
On Thu, Mar 09, 2023 at 08:48:28PM +0800, Huang, Ying wrote:
> Yosry Ahmed <yosryahmed@google.com> writes:
>
> >
> > struct swap_desc {
> > union { /* Use one bit to distinguish them */
> > swp_entry_t swap_entry;
> > struct zswap_entry *zswap_entry;
> > };
> > struct folio *swapcache;
> > atomic_t swap_count;
> > u32 id;
> > }
> >
> > Having the id in the swap_desc is convenient as we can directly map
> > the swap_desc to a swp_entry_t to place in the page tables, but I
> > don't think it's necessary. Without it, the struct size is 20 bytes,
> > so I think the extra 4 bytes are okay to use anyway if the slab
> > allocator only allocates multiples of 8 bytes.
> >
> > The idea here is to unify the swapcache and swap_count implementation
> > between different swap backends (swapfiles, zswap, etc), which would
> > create a better abstraction and reduce reinventing the wheel.
> >
> > We can reduce to only 8 bytes and only store the swap/zswap entry, but
> > we still need the swap cache anyway so might as well just store the
> > pointer in the struct and have a unified lookup-free swapcache, so
> > really 16 bytes is the minimum.
> >
> > If we stop at 16 bytes, then we need to handle swap count separately
> > in swapfiles and zswap. This is not the end of the world, but are the
> > 8 bytes worth this?
>
> If my understanding were correct, for current implementation, we need
> one swap cache pointer per swapped out page too. Even after calling
> __delete_from_swap_cache(), we store the "shadow" entry there. Although
That is correct. We have the "shadow" entry.
> it's possible to implement shadow entry reclaiming like that for file
> cache shadow entry (workingset_shadow_shrinker), we haven't done that
> yet. And, it appears that we can live with that. So, in current
> implementation, for each swapped out page, we use 9 bytes. If so, the
> memory usage ratio is 24 / 9 = 2.667, still not trivial, but not as
> horrible as 24 / 1 = 24.
The swap_desc proposal did not explicit save the shadow entry in swap_desc.
So the math should be (24 + 8) vs ( 1 + 8). There is about 20 byte extra
per page frame.
> > Instead, if we store a key else in swp_entry_t and use this to lookup
> > the swp_entry_t or zswap_entry pointer then that's essentially what
> > the swap_desc does. It just goes the extra mile of unifying the
> > swapcache as well and storing it directly in the swap_desc instead of
> > storing it in another lookup structure.
>
> If we choose to make sizeof(struct swap_desc) == 8, that is, store only
> swap_entry in swap_desc. The added indirection appears to be another
> level of page table with 1 entry. Then, we may use the similar method
> as supporting system with 2 level and 3 level page tables, like the code
> in include/asm-generic/pgtable-nopmd.h. But I haven't thought about
> this deeply.
I would like to explore other possibility as well. More idea and discussion is
welcome.
Chris
next prev parent reply other threads:[~2023-03-09 19:59 UTC|newest]
Thread overview: 105+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-02-18 22:38 [LSF/MM/BPF TOPIC] Swap Abstraction / Native Zswap Yosry Ahmed
2023-02-19 4:31 ` Matthew Wilcox
2023-02-19 9:34 ` Yosry Ahmed
2023-02-28 23:22 ` Chris Li
2023-03-01 0:08 ` Matthew Wilcox
2023-03-01 23:22 ` Chris Li
2023-02-21 18:39 ` Yang Shi
2023-02-21 18:56 ` Yosry Ahmed
2023-02-21 19:26 ` Yang Shi
2023-02-21 19:46 ` Yosry Ahmed
2023-02-21 23:34 ` Yang Shi
2023-02-21 23:38 ` Yosry Ahmed
2023-02-22 16:57 ` Johannes Weiner
2023-02-22 22:46 ` Yosry Ahmed
2023-02-28 4:29 ` Kalesh Singh
2023-02-28 8:09 ` Yosry Ahmed
2023-02-28 4:54 ` Sergey Senozhatsky
2023-02-28 8:12 ` Yosry Ahmed
2023-02-28 23:29 ` Minchan Kim
2023-03-02 0:58 ` Yosry Ahmed
2023-03-02 1:25 ` Yosry Ahmed
2023-03-02 17:05 ` Chris Li
2023-03-02 17:47 ` Chris Li
2023-03-02 18:15 ` Johannes Weiner
2023-03-02 18:56 ` Chris Li
2023-03-02 18:23 ` Rik van Riel
2023-03-02 21:42 ` Chris Li
2023-03-02 22:36 ` Rik van Riel
2023-03-02 22:55 ` Yosry Ahmed
2023-03-03 4:05 ` Chris Li
2023-03-03 0:01 ` Chris Li
2023-03-02 16:58 ` Chris Li
2023-03-01 10:44 ` Sergey Senozhatsky
2023-03-02 1:01 ` Yosry Ahmed
2023-02-28 23:11 ` Chris Li
2023-03-02 0:30 ` Yosry Ahmed
2023-03-02 1:00 ` Yosry Ahmed
2023-03-02 16:51 ` Chris Li
2023-03-03 0:33 ` Minchan Kim
2023-03-03 0:49 ` Yosry Ahmed
2023-03-03 1:25 ` Minchan Kim
2023-03-03 17:15 ` Yosry Ahmed
2023-03-09 12:48 ` Huang, Ying
2023-03-09 19:58 ` Chris Li [this message]
2023-03-09 20:19 ` Yosry Ahmed
2023-03-10 3:06 ` Huang, Ying
2023-03-10 23:14 ` Chris Li
2023-03-13 1:10 ` Huang, Ying
2023-03-15 7:41 ` Yosry Ahmed
2023-03-16 1:42 ` Huang, Ying
2023-03-11 1:06 ` Yosry Ahmed
2023-03-13 2:12 ` Huang, Ying
2023-03-15 8:01 ` Yosry Ahmed
2023-03-16 7:50 ` Huang, Ying
2023-03-17 10:19 ` Yosry Ahmed
2023-03-17 18:19 ` Chris Li
2023-03-17 18:23 ` Yosry Ahmed
2023-03-20 2:55 ` Huang, Ying
2023-03-20 6:25 ` Chris Li
2023-03-23 0:56 ` Huang, Ying
2023-03-23 6:46 ` Chris Li
2023-03-23 6:56 ` Huang, Ying
2023-03-23 18:28 ` Chris Li
2023-03-23 18:40 ` Yosry Ahmed
2023-03-23 19:49 ` Chris Li
2023-03-23 19:54 ` Yosry Ahmed
2023-03-23 21:10 ` Chris Li
2023-03-24 17:28 ` Chris Li
2023-03-22 5:56 ` Yosry Ahmed
2023-03-23 1:48 ` Huang, Ying
2023-03-23 2:21 ` Yosry Ahmed
2023-03-23 3:16 ` Huang, Ying
2023-03-23 3:27 ` Yosry Ahmed
2023-03-23 5:37 ` Huang, Ying
2023-03-23 15:18 ` Yosry Ahmed
2023-03-24 2:37 ` Huang, Ying
2023-03-24 7:28 ` Yosry Ahmed
2023-03-24 17:23 ` Chris Li
2023-03-27 1:23 ` Huang, Ying
2023-03-28 5:54 ` Yosry Ahmed
2023-03-28 6:20 ` Huang, Ying
2023-03-28 6:29 ` Yosry Ahmed
2023-03-28 6:59 ` Huang, Ying
2023-03-28 7:59 ` Yosry Ahmed
2023-03-28 14:14 ` Johannes Weiner
2023-03-28 19:59 ` Yosry Ahmed
2023-03-28 21:22 ` Chris Li
2023-03-28 21:30 ` Yosry Ahmed
2023-03-28 20:50 ` Chris Li
2023-03-28 21:01 ` Yosry Ahmed
2023-03-28 21:32 ` Chris Li
2023-03-28 21:44 ` Yosry Ahmed
2023-03-28 22:01 ` Chris Li
2023-03-28 22:02 ` Yosry Ahmed
2023-03-29 1:31 ` Huang, Ying
2023-03-29 1:41 ` Yosry Ahmed
2023-03-29 16:04 ` Chris Li
2023-04-04 8:24 ` Huang, Ying
2023-04-04 8:10 ` Huang, Ying
2023-04-04 8:47 ` Yosry Ahmed
2023-04-06 1:40 ` Huang, Ying
2023-03-29 15:22 ` Chris Li
2023-03-10 2:07 ` Luis Chamberlain
2023-03-10 2:15 ` Yosry Ahmed
2023-05-12 3:07 ` Yosry Ahmed
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