Chris Li <chrisl@xxxxxxxxxx> writes: > On Mon, Jun 24, 2024 at 7:36 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote: >> >> Chris Li <chrisl@xxxxxxxxxx> writes: >> >> > On Wed, Jun 19, 2024 at 7:32 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote: >> >> >> >> Chris Li <chrisl@xxxxxxxxxx> writes: >> >> >> >> > This is the short term solutiolns "swap cluster order" listed >> >> > in my "Swap Abstraction" discussion slice 8 in the recent >> >> > LSF/MM conference. >> >> > >> >> > When commit 845982eb264bc "mm: swap: allow storage of all mTHP >> >> > orders" is introduced, it only allocates the mTHP swap entries >> >> > from new empty cluster list. It has a fragmentation issue >> >> > reported by Barry. >> >> > >> >> > https://lore.kernel.org/all/CAGsJ_4zAcJkuW016Cfi6wicRr8N9X+GJJhgMQdSMp+Ah+NSgNQ@xxxxxxxxxxxxxx/ >> >> > >> >> > The reason is that all the empty cluster has been exhausted while >> >> > there are planty of free swap entries to in the cluster that is >> >> > not 100% free. >> >> > >> >> > Remember the swap allocation order in the cluster. >> >> > Keep track of the per order non full cluster list for later allocation. >> >> > >> >> > User impact: For users that allocate and free mix order mTHP swapping, >> >> > It greatly improves the success rate of the mTHP swap allocation after the >> >> > initial phase. >> >> > >> >> > Barry provides a test program to show the effect: >> >> > https://lore.kernel.org/linux-mm/20240615084714.37499-1-21cnbao@xxxxxxxxx/ >> >> > >> >> > Without: >> >> > $ mthp-swapout >> >> > Iteration 1: swpout inc: 222, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 2: swpout inc: 219, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 3: swpout inc: 222, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 4: swpout inc: 219, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 5: swpout inc: 110, swpout fallback inc: 117, Fallback percentage: 51.54% >> >> > Iteration 6: swpout inc: 0, swpout fallback inc: 230, Fallback percentage: 100.00% >> >> > Iteration 7: swpout inc: 0, swpout fallback inc: 229, Fallback percentage: 100.00% >> >> > Iteration 8: swpout inc: 0, swpout fallback inc: 223, Fallback percentage: 100.00% >> >> > Iteration 9: swpout inc: 0, swpout fallback inc: 224, Fallback percentage: 100.00% >> >> > Iteration 10: swpout inc: 0, swpout fallback inc: 216, Fallback percentage: 100.00% >> >> > Iteration 11: swpout inc: 0, swpout fallback inc: 212, Fallback percentage: 100.00% >> >> > Iteration 12: swpout inc: 0, swpout fallback inc: 224, Fallback percentage: 100.00% >> >> > Iteration 13: swpout inc: 0, swpout fallback inc: 214, Fallback percentage: 100.00% >> >> > >> >> > $ mthp-swapout -s >> >> > Iteration 1: swpout inc: 222, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 2: swpout inc: 227, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 3: swpout inc: 222, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 4: swpout inc: 224, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 5: swpout inc: 33, swpout fallback inc: 197, Fallback percentage: 85.65% >> >> > Iteration 6: swpout inc: 0, swpout fallback inc: 229, Fallback percentage: 100.00% >> >> > Iteration 7: swpout inc: 0, swpout fallback inc: 223, Fallback percentage: 100.00% >> >> > Iteration 8: swpout inc: 0, swpout fallback inc: 219, Fallback percentage: 100.00% >> >> > Iteration 9: swpout inc: 0, swpout fallback inc: 212, Fallback percentage: 100.00% >> >> > >> >> > With: >> >> > $ mthp-swapout >> >> > Iteration 1: swpout inc: 222, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 2: swpout inc: 219, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 3: swpout inc: 222, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 4: swpout inc: 219, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 5: swpout inc: 227, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 6: swpout inc: 230, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > ... >> >> > Iteration 94: swpout inc: 224, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 95: swpout inc: 221, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 96: swpout inc: 229, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 97: swpout inc: 219, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 98: swpout inc: 222, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 99: swpout inc: 223, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 100: swpout inc: 224, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > >> >> > $ mthp-swapout -s >> >> > Iteration 1: swpout inc: 222, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 2: swpout inc: 227, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 3: swpout inc: 222, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 4: swpout inc: 224, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 5: swpout inc: 230, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 6: swpout inc: 229, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 7: swpout inc: 223, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 8: swpout inc: 219, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > ... >> >> > Iteration 94: swpout inc: 223, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 95: swpout inc: 212, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 96: swpout inc: 220, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 97: swpout inc: 220, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 98: swpout inc: 216, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 99: swpout inc: 223, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> > Iteration 100: swpout inc: 225, swpout fallback inc: 0, Fallback percentage: 0.00% >> >> >> >> Unfortunately, the data is gotten using a special designed test program >> >> which always swap-in pages with swapped-out size. I don't know whether >> >> such workloads exist in reality. Otherwise, you need to wait for mTHP >> > >> > The test program is designed to simulate mTHP swap behavior using >> > zsmalloc and 64KB buffer. >> > If we insist on only designing for existing workloads, then zsmalloc >> > using 64KB buffer usage will never be able to run, exactly due the >> > kernel has high failure rate allocating swap entries for 64KB. There >> > is a bit of a chick and egg problem there, such a usage can not exist >> > because the kernel can't support it yet. Kernel can't add patches to >> > support it because such simulation tests are not "real". >> > >> > We need to break this cycle to support something new. >> > >> >> swap-in to be merged firstly, and people reach consensus that we should >> >> always swap-in pages with swapped-out size. >> > >> > We don't have to be always. We can identify the situation that makes >> > sense. For the zram/zsmalloc 64K buffer usage case, swap out as the >> > same swap in size makes sense. >> > I think we have agreement on such zsmalloc 64K usage cases we do want >> > to support. >> > >> >> >> >> Alternately, we can make some design adjustment to make the patchset >> >> work in current situation (mTHP swap-out, normal page swap-in). >> >> >> >> - One non-full cluster list for each order (same as current design) >> >> >> >> - When one swap entry is freed, check whether one "order+1" swap entry >> >> becomes free, if so, move the cluster to "order+1" non-full cluster >> >> list. >> > >> > In the intended zsmalloc usage case, there is no order+1 swap entry >> > request. >> >> This my main concern about this series. Only the Android use cases are >> considered. The general use cases are just ignored. Is it hard to >> consider or test a normal swap partition on your development machine? > > Please see the V4 cover letter. The V4 already has the SSD, zram and > HDD stress testing. > Of course I want to make sure the allocator works well with Barry's > mthp test case as well. > >> > Moving the cluster to "order+1" will make less cluster available for "order". >> > For that usage case it is negative gain. >> >> The "order+1" cluster can be used to allocate "order" cluster when >> existing "order" cluster is used up. >> >> And in this way, we can protect clusters with more free spaces so that >> they may become free. >> >> >> - When allocate swap entry with "order", get cluster from free, "order", >> >> "order+1", ... non-full cluster list. If all are empty, fallback to >> > >> > I don't see that it is useful for the zsmalloc 64K buffer usage case. >> > There will be order 0 and order 4 and nothing else. >> > >> > How about let's keep it simple for now. If we identify some workload >> > this algorithm can help. We can do that as a follow up step. >> >> The simple design isn't flexible enough for your workloads too. For >> example, >> >> - Initially, almost only order-0 pages are swapped out, most non-full >> clusters are order-0. >> >> - Later, quite some order-0 swap entries are freed so that there are >> quite some order-4 swap entries available. >> >> - Order-4 pages need to be swapped out, but no enough order-4 non-full >> clusters available. >> >> So, we need a way to migrate non-full clusters among orders to adjust to >> the situations automatically. > > Depends on how lucky it is to form the order-4 cluster naturally. The > odds of forming the order-4 cluster naturally in random swap > allocation/ free case is very low. I have the number in my other email > thread. > Anyway, if we convince this payout for the complexity it introduces, > we can do that as follow up steps. Try to keep things simple at first > for the review benefit. > >> >> >> order 0. >> >> >> >> Do you think that this works? >> >> >> >> > Reported-by: Barry Song <21cnbao@xxxxxxxxx> >> >> > Signed-off-by: Chris Li <chrisl@xxxxxxxxxx> >> >> > --- >> >> > Changes in v3: >> >> > - Using V1 as base. >> >> > - Rename "next" to "list" for the list field, suggested by Ying. >> >> > - Update comment for the locking rules for cluster fields and list, >> >> > suggested by Ying. >> >> > - Allocate from the nonfull list before attempting free list, suggested >> >> > by Kairui. >> >> >> >> Haven't looked into this. It appears that this breaks the original >> >> discard behavior which helps performance of some SSD, please refer to >> > >> > Can you clarify by "discard" you mean SSD discard command or just the >> > way swap allocator recycles free clusters? >> >> The SSD discard command, like in the following URL, >> >> https://en.wikipedia.org/wiki/Trim_(computing) > > Thanks. I know what an SSD discard command is. Want to understand why > that behavior is preferred. > > So the reasoning to prefer a new free block rather than a recent > particle free cluster is to let the previous written cluster have a > higher chance to issue the discard command? > > This preferred new block behavior is actually not friendly to SSD from > a wearing point of view. > Take this example: > Let say the data need to allocate and free from swap. At any given > time the swap usage is 1G. The swap SSD drive is 16G. > Let say the allocation and free are at random 4K page locations. There > is totally 64G swap data needed to write to swap, but at any given > time there is only 1G data occupite on swapfile. > > a) If you always prefer new free blocks. Then the swap data will > eventually write at all 16G drives then random write to full 16G. > Chance of forming a free cluster so a discard command can be issued is > very low. (15/16)**512 = 4.4E-15. From SSD point of view, it does not > know most of the data written to 16G drive is not used. When a page is > free on a swapfile, SSD drive doesn't know about it. It sees 4K random > writes to all 16G of the drive, total 64G data written. > > b) If you always prefer a non full cluster first over a new cluster. > The 64G data will concentrate random writing to the first 1G of drive > location. Total 64G data written. > > I consider b) are more friendly to SSD than a). Because concentrate > the write into the first 1G location. The SSD can know the data > overwritten in those 1G has internally obsolete, so it can internally > GC the those overwritten data without a discard command. Where a) > random 4K writes to the whole drive without much discard at all. Full > SSD doing random writes is a bad combination from a wearing point of > view. > > Just my 2 cents. Anyway I revert the V4 to use free cluster before > nonfull cluster just to behave the same as previously. > >> >> commit 2a8f94493432 ("swap: change block allocation algorithm for SSD"). >> > >> > I did read that change log. Help me understand in more detail which >> > discard behavior you have in mind. A lot of low end micro SD cards >> > have proper FTL wear leveling now, ssd even better on that. >> >> It's not FTL, it's discard/trim for SSD as above. > > Thanks for the clarification. > >> >> >> And as pointed out by Ryan, this may reduce the opportunity of the >> >> sequential block device writing during swap-out, which may hurt >> >> performance of SSD too. >> > >> > Only at the initial phase. If the swap IO continues, after the first >> > pass fills up the swap file, the write will be random on the swapfile >> > anyway. Because the swapfile only issues 2M discards commands when all >> > 512 4K pages are free. The discarded area will be much smaller than >> > the free area on swapfile. That combined with the random write page on >> > the whole swap file. It might produce a worse internal write >> > amplification for SSD, compared to only writing a subset of the >> > swapfile area. I would love to hear from someone who understands SSD >> > internals to confirm or deny my theory. >> >> It depends on workloads. Some workloads will have more severe >> fragmentation than others. For example, on quite some machines, the >> swap devices will be far from being full to avoid possible OOM. > > I suspect most of the SSD swap on client devices nowadays are only as > backup just in case it needs to be swapped. > There is not much SSD swap IO during normal use. The zram and zswap > are more actively used in the data center and Android phone case, from > swap IO ops point of view. I use a Linux laptop with 16GB DRAM for work. And I found that the swap space are almost always used. >> >> > Even let's assume the SSD wants a free block over a nonfull cluster >> > first. Zswap and zram swap are not subject to SSD property. We might >> > want to have a kernel option to select using nonfree clusters over >> > the free one for zram and zswap (ghost swapfile). That will help >> > contain the fragmented swap area. >> >> I suspect that it will help fragmentation avoidance much. Please prove >> its effectiveness with data firstly. It can be a further optimization >> patch in the series. > > Take the above 1GB data written in a 16GB drive example. a) will > fragment the whole 16GB drive. > b) will concentrate on the first 1GB location that was used. > >> >> Even if we really need it, we can try to do it without a kernel option. >> For example, detect whether we are using zram and enable it for zram >> automatically (through a general flag). > > zswap you need to have an option to choose from because it can write > to the real swappfile as well. > Do you optimize the swap allocator for the zswap or physical swapfile. -- Best Regards, Huang, Ying
