Yosry Ahmed <yosryahmed@xxxxxxxxxx> writes:
> On Tue, Feb 28, 2023 at 3:11 PM Chris Li <chrisl@xxxxxxxxxx> 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?
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
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.
> 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
> continuation pages. If we do, it may end up being more. We also
> allocate continuation in full 4k pages, so even if one swap_map
> element in a page requires continuation, we will allocate an entire
> page. What I am trying to say is that to get an actual comparison you
> need to also factor in the swap utilization and the rate of usage of
> swap continuation. I don't know how to come up with a formula for this
> tbh.
>
> Also, like Johannes said, the worst case overhead (32 bytes if you
> count the reverse mapping) is 0.8% of swapped memory, aka 8M for every
> 1G swapped. It doesn't sound *very* bad. I understand that it is pure
> overhead for people not using zswap, but it is not very awful.
>
>>
>> It seems what you really need is one bit of information to indicate
>> this page is backed by zswap. Then you can have a seperate pointer
>> for the zswap entry.
>
> If you use one bit in swp_entry_t (or one of the available swap types)
> to indicate whether the page is backed with a swapfile or zswap it
> doesn't really work. We lose the indirection layer. How do we move the
> page from zswap to swapfile? We need to go update the page tables and
> the shmem page cache, similar to swapoff.
>
> 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.
>>
>> Depending on how much you are going to reuse the swap cache, you might
>> need to have something like a swap_info_struct to keep the locks happy.
>
> My current intention is to reimplement the swapcache completely as a
> pointer in struct swap_desc. This would eliminate this need and a lot
> of the locking we do today if I get things right.
>
>>
>> > Another potential concern is readahead. With this design, we have no
>>
>> Readahead is for spinning disk :-) Even a normal swap file with an SSD can
>> use some modernization.
>
> Yeah, I initially thought we would only need the swp_entry_t ->
> swap_desc reverse mapping for readahead, and that we can only store
> that for spinning disks, but I was wrong. We need for other things as
> well today: swapoff, when trying to find an empty swap slot and we
> start trying to free swap slots used only by the swapcache. However, I
> think both of these cases can be fixed (I can share more details if
> you want). If everything goes well we should only need to maintain the
> reverse mapping (extra overhead above 24 bytes) for swap files on
> spinning disks for readahead.
>
>>
>> Looking forward to your discussion.
>>
>> Chris
>>
Best Regards,
Huang, Ying
