From: Yosry Ahmed <yosryahmed@google.com>
To: Minchan Kim <minchan@kernel.org>
Cc: Chris Li <chrisl@kernel.org>,
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>,
Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [LSF/MM/BPF TOPIC] Swap Abstraction / Native Zswap
Date: Thu, 2 Mar 2023 16:49:01 -0800 [thread overview]
Message-ID: <CAJD7tkbMGJ2C79jwXs8m8LMpDGe1xFw4_uMmCgONc17WMX53Cw@mail.gmail.com> (raw)
In-Reply-To: <ZAFAWPXX8QGkep0G@google.com>
On Thu, Mar 2, 2023 at 4:33 PM Minchan Kim <minchan@kernel.org> wrote:
>
> On Wed, Mar 01, 2023 at 04:30:22PM -0800, Yosry Ahmed wrote:
> > On Tue, Feb 28, 2023 at 3:11 PM Chris Li <chrisl@kernel.org> wrote:
> > >
> > > Hi Yosry,
> > >
> > > On Sat, Feb 18, 2023 at 02:38:40PM -0800, Yosry Ahmed wrote:
> > > > Hello everyone,
> > > >
> > > > I would like to propose a topic for the upcoming LSF/MM/BPF in May
> > > > 2023 about swap & zswap (hope I am not too late).
> > >
> > > I am very interested in participating in this discussion as well.
> >
> > That's great to hear!
> >
> > >
> > > > ==================== Objective ====================
> > > > Enabling the use of zswap without a backing swapfile, which makes
> > > > zswap useful for a wider variety of use cases. Also, when zswap is
> > > > used with a swapfile, the pages in zswap do not use up space in the
> > > > swapfile, so the overall swapping capacity increases.
> > >
> > > Agree.
> > >
> > > >
> > > > ==================== Idea ====================
> > > > Introduce a data structure, which I currently call a swap_desc, as an
> > > > abstraction layer between swapping implementation and the rest of MM
> > > > code. Page tables & page caches would store a swap id (encoded as a
> > > > swp_entry_t) instead of directly storing the swap entry associated
> > > > with the swapfile. This swap id maps to a struct swap_desc, which acts
> > >
> > > Can you provide a bit more detail? I am curious how this swap id
> > > maps into the swap_desc? Is the swp_entry_t cast into "struct
> > > swap_desc*" or going through some lookup table/tree?
> >
> > swap id would be an index in a radix tree (aka xarray), which contains
> > a pointer to the swap_desc struct. This lookup should be free with
> > this design as we also use swap_desc to directly store the swap cache
> > pointer, so this lookup essentially replaces the swap cache lookup.
> >
> > >
> > > > as our abstraction layer. All MM code not concerned with swapping
> > > > details would operate in terms of swap descs. The swap_desc can point
> > > > to either a normal swap entry (associated with a swapfile) or a zswap
> > > > entry. It can also include all non-backend specific operations, such
> > > > as the swapcache (which would be a simple pointer in swap_desc), swap
> > >
> > > Does the zswap entry still use the swap slot cache and swap_info_struct?
> >
> > In this design no, it shouldn't.
> >
> > >
> > > > This work enables using zswap without a backing swapfile and increases
> > > > the swap capacity when zswap is used with a swapfile. It also creates
> > > > a separation that allows us to skip code paths that don't make sense
> > > > in the zswap path (e.g. readahead). We get to drop zswap's rbtree
> > > > which might result in better performance (less lookups, less lock
> > > > contention).
> > > >
> > > > The abstraction layer also opens the door for multiple cleanups (e.g.
> > > > removing swapper address spaces, removing swap count continuation
> > > > code, etc). Another nice cleanup that this work enables would be
> > > > separating the overloaded swp_entry_t into two distinct types: one for
> > > > things that are stored in page tables / caches, and for actual swap
> > > > entries. In the future, we can potentially further optimize how we use
> > > > the bits in the page tables instead of sticking everything into the
> > > > current type/offset format.
> > >
> > > Looking forward to seeing more details in the upcoming discussion.
> > > >
> > > > ==================== Cost ====================
> > > > The obvious downside of this is added memory overhead, specifically
> > > > for users that use swapfiles without zswap. Instead of paying one byte
> > > > (swap_map) for every potential page in the swapfile (+ swap count
> > > > continuation), we pay the size of the swap_desc for every page that is
> > > > actually in the swapfile, which I am estimating can be roughly around
> > > > 24 bytes or so, so maybe 0.6% of swapped out memory. The overhead only
> > > > scales with pages actually swapped out. For zswap users, it should be
> > >
> > > Is there a way to avoid turning 1 byte into 24 byte per swapped
> > > pages? For the users that use swap but no zswap, this is pure overhead.
> >
> > That's what I could think of at this point. My idea was something like this:
> >
> > 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?
> >
> > Keep in mind that the current overhead is 1 byte O(max swap pages) not
> > O(swapped). Also, 1 byte is assuming we do not use the swap
>
> Just to share info:
>
> Android usually used swap space fully most of times via Compacting
> background Apps so O(swapped) ~= O(max swap pages).
Thanks for sharing this, that's definitely interesting.
What percentage of memory is usually provisioned as swap in such
cases? Would you consider an extra overhead of ~8M per 1G of swapped
memory particularly unacceptable?
next prev parent reply other threads:[~2023-03-03 0:49 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 [this message]
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
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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