From: Yosry Ahmed <yosryahmed@google.com>
To: lsf-pc@lists.linux-foundation.org, Johannes Weiner <hannes@cmpxchg.org>
Cc: 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, 11 May 2023 20:07:36 -0700 [thread overview]
Message-ID: <CAJD7tkYb_sGN8mfGVjr2JxdB8Pz8Td=yj9_sBCMrmsKQo56vTg@mail.gmail.com> (raw)
In-Reply-To: <CAJD7tkbCnXJ95Qow_aOjNX6NOMU5ovMSHRC+95U4wtW6cM+puw@mail.gmail.com>
[-- Attachment #1: Type: text/plain, Size: 5184 bytes --]
On Sat, Feb 18, 2023 at 2:38 PM Yosry Ahmed <yosryahmed@google.com> 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).
>
> ==================== Intro ====================
> Currently, using zswap is dependent on swapfiles in an unnecessary
> way. To use zswap, you need a swapfile configured (even if the space
> will not be used) and zswap is restricted by its size. When pages
> reside in zswap, the corresponding swap entry in the swapfile cannot
> be used, and is essentially wasted. We also go through unnecessary
> code paths when using zswap, such as finding and allocating a swap
> entry on the swapout path, or readahead in the swapin path. I am
> proposing a swapping abstraction layer that would allow us to remove
> zswap's dependency on swapfiles. This can be done by introducing a
> data structure between the actual swapping implementation (swapfiles,
> zswap) and the rest of the MM code.
>
> ==================== 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.
>
> ==================== 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
> 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
> counting, etc. It creates a clear, nice abstraction layer between MM
> code and the actual swapping implementation.
>
> ==================== Benefits ====================
> 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.
>
> Another potential win here can be swapoff, which can be more practical
> by directly scanning all swap_desc's instead of going through page
> tables and shmem page caches.
>
> Overall zswap becomes more accessible and available to a wider range
> of use cases.
>
> ==================== 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
> a win (or at least even) because we get to drop a lot of fields from
> struct zswap_entry (e.g. rbtree, index, etc).
>
> Another potential concern is readahead. With this design, we have no
> way to get a swap_desc given a swap entry (type & offset). We would
> need to maintain a reverse mapping, adding a little bit more overhead,
> or search all swapped out pages instead :). A reverse mapping might
> pump the per-swapped page overhead to ~32 bytes (~0.8% of swapped out
> memory).
>
> ==================== Bottom Line ====================
> It would be nice to discuss the potential here and the tradeoffs. I
> know that other folks using zswap (or interested in using it) may find
> this very useful. I am sure I am missing some context on why things
> are the way they are, and perhaps some obvious holes in my story.
> Looking forward to discussing this with anyone interested :)
>
> I think Johannes may be interested in attending this discussion, since
> a lot of ideas here are inspired by discussions I had with him :)
For the record, here are the slides that were presented for this
discussion (attached).
[-- Attachment #2: [LSF_MM_BPF 2023] Swap Abstraction _ Native Zswap.pdf --]
[-- Type: application/pdf, Size: 133190 bytes --]
prev parent reply other threads:[~2023-05-12 3:08 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
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 [this message]
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