On 01.03.24 17:27, Ryan Roberts wrote:
On 28/02/2024 15:12, David Hildenbrand wrote:
On 28.02.24 15:57, Ryan Roberts wrote:
On 28/02/2024 12:12, David Hildenbrand wrote:
How relevant is it? Relevant enough that someone decided to put that
optimization in? I don't know :)
I'll have one last go at convincing you: Huang Ying (original author) commented
"I believe this should be OK. Better to compare the performance too." at [1].
That implies to me that perhaps the optimization wasn't in response to a
specific problem after all. Do you have any thoughts, Huang?
Might make sense to include that in the patch description!
OK so if we really do need to keep this optimization, here are some ideas:
Fundamentally, we would like to be able to figure out the size of the swap slot
from the swap entry. Today swap supports 2 sizes; PAGE_SIZE and PMD_SIZE. For
PMD_SIZE, it always uses a full cluster, so can easily add a flag to the
cluster
to mark it as PMD_SIZE.
Going forwards, we want to support all sizes (power-of-2). Most of the time, a
cluster will contain only one size of THPs, but this is not the case when a THP
in the swapcache gets split or when an order-0 slot gets stolen. We expect
these
cases to be rare.
1) Keep the size of the smallest swap entry in the cluster header. Most of the
time it will be the full size of the swap entry, but sometimes it will cover
only a portion. In the latter case you may see a false negative for
swap_page_trans_huge_swapped() meaning we take the slow path, but that is rare.
There is one wrinkle: currently the HUGE flag is cleared in
put_swap_folio(). We
wouldn't want to do the equivalent in the new scheme (i.e. set the whole
cluster
to order-0). I think that is safe, but haven't completely convinced myself yet.
2) allocate 4 bits per (small) swap slot to hold the order. This will give
precise information and is conceptually simpler to understand, but will cost
more memory (half as much as the initial swap_map[] again).
I still prefer to avoid this at all if we can (and would like to hear Huang's
thoughts). But if its a choice between 1 and 2, I prefer 1 - I'll do some
prototyping.
Taking a step back: what about we simply batch unmapping of swap entries?
That is, if we're unmapping a PTE range, we'll collect swap entries (under PT
lock) that reference consecutive swap offsets in the same swap file.
Yes in principle, but there are 4 places where free_swap_and_cache() is called,
and only 2 of those are really amenable to batching (zap_pte_range() and
madvise_free_pte_range()). So the other two users will still take the "slow"
path. Maybe those 2 callsites are the only ones that really matter? I can
certainly have a stab at this approach.
We can ignore the s390x one. That s390x code should only apply to KVM guest
memory where ordinary THP are not even supported. (and nobody uses mTHP there yet).
Long story short: the VM can hint that some memory pages are now unused and the
hypervisor can reclaim them. That's what that callback does (zap guest-provided
guest memory). No need to worry about any batching for now.
Then, there is the shmem one in shmem_free_swap(). I really don't know how shmem
handles THP+swapout.
But looking at shmem_writepage(), we split any large folios before moving them
to the swapcache, so likely we don't care at all, because THP don't apply.
There, we can then first decrement all the swap counts, and then try minimizing
how often we actually have to try reclaiming swap space (lookup folio, see it's
a large folio that we cannot reclaim or could reclaim, ...).
Might need some fine-tuning in swap code to "advance" to the next entry to try
freeing up, but we certainly can do better than what we would do right now.
I'm not sure I've understood this. Isn't advancing just a matter of:
entry = swp_entry(swp_type(entry), swp_offset(entry) + 1);
I was talking about the advancing swapslot processing after decrementing the
swapcounts.
Assume you decremented 512 swapcounts and some of them went to 0. AFAIU, you'd
have to start with the first swapslot that has now a swapcount=0 one and try to
reclaim swap.
Assume you get a small folio, then you'll have to proceed with the next swap
slot and try to reclaim swap.
Assume you get a large folio, then you can skip more swapslots (depending on
offset into the folio etc).
If you get what I mean. :)
I've implemented the batching as David suggested, and I'm pretty confident it's
correct. The only problem is that during testing I can't provoke the code to
take the path. I've been pouring through the code but struggling to figure out
under what situation you would expect the swap entry passed to
free_swap_and_cache() to still have a cached folio? Does anyone have any idea?
This is the original (unbatched) function, after my change, which caused David's
concern that we would end up calling __try_to_reclaim_swap() far too much:
int free_swap_and_cache(swp_entry_t entry)
{
struct swap_info_struct *p;
unsigned char count;
if (non_swap_entry(entry))
return 1;
p = _swap_info_get(entry);
if (p) {
count = __swap_entry_free(p, entry);
if (count == SWAP_HAS_CACHE)
__try_to_reclaim_swap(p, swp_offset(entry),
TTRS_UNMAPPED | TTRS_FULL);
}
return p != NULL;
}
The trouble is, whenever its called, count is always 0, so
__try_to_reclaim_swap() never gets called.
My test case is allocating 1G anon memory, then doing madvise(MADV_PAGEOUT) over
it. Then doing either a munmap() or madvise(MADV_FREE), both of which cause this
function to be called for every PTE, but count is always 0 after
__swap_entry_free() so __try_to_reclaim_swap() is never called. I've tried for
order-0 as well as PTE- and PMD-mapped 2M THP.
I'm guessing the swapcache was already reclaimed as part of MADV_PAGEOUT? I'm
using a block ram device as my backing store - I think this does synchronous IO
so perhaps if I have a real block device with async IO I might have more luck?
Just a guess...
Or perhaps this code path is a corner case? In which case, perhaps its not worth
adding the batching optimization after all?
I had to disable zswap in the past and was able to trigger this reliably
with an ordinary swap backend (e.g., proper disk).
Whenever you involve swap-to-ram, you might just get it reclaimed
immediately.
--
Cheers,
David / dhildenb
