On Thu, Mar 28, 2024 at 09:27:17PM -0700, Yosry Ahmed wrote: > On Thu, Mar 28, 2024 at 7:05 PM Yosry Ahmed <yosryahmed@xxxxxxxxxx> wrote: > > > > On Thu, Mar 28, 2024 at 4:19 PM Nhat Pham <nphamcs@xxxxxxxxx> wrote: > > > > > > On Thu, Mar 28, 2024 at 2:07 PM Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > > > > > > > > On Thu, Mar 28, 2024 at 01:23:42PM -0700, Yosry Ahmed wrote: > > > > > On Thu, Mar 28, 2024 at 12:31 PM Johannes Weiner <hannes@xxxxxxxxxxx> wrote: > > > > > > > > > > > > On Mon, Mar 25, 2024 at 11:50:14PM +0000, Yosry Ahmed wrote: > > > > > > > The current same-filled pages handling supports pages filled with any > > > > > > > repeated word-sized pattern. However, in practice, most of these should > > > > > > > be zero pages anyway. Other patterns should be nearly as common. > > > > > > > > > > > > > > Drop the support for non-zero same-filled pages, but keep the names of > > > > > > > knobs exposed to userspace as "same_filled", which isn't entirely > > > > > > > inaccurate. > > > > > > > > > > > > > > This yields some nice code simplification and enables a following patch > > > > > > > that eliminates the need to allocate struct zswap_entry for those pages > > > > > > > completely. > > > > > > > > > > > > > > There is also a very small performance improvement observed over 50 runs > > > > > > > of kernel build test (kernbench) comparing the mean build time on a > > > > > > > skylake machine when building the kernel in a cgroup v1 container with a > > > > > > > 3G limit: > > > > > > > > > > > > > > base patched % diff > > > > > > > real 70.167 69.915 -0.359% > > > > > > > user 2953.068 2956.147 +0.104% > > > > > > > sys 2612.811 2594.718 -0.692% > > > > > > > > > > > > > > This probably comes from more optimized operations like memchr_inv() and > > > > > > > clear_highpage(). Note that the percentage of zero-filled pages during > > > > > > > this test was only around 1.5% on average, and was not affected by this > > > > > > > patch. Practical workloads could have a larger proportion of such pages > > > > > > > (e.g. Johannes observed around 10% [1]), so the performance improvement > > > > > > > should be larger. > > > > > > > > > > > > > > [1]https://lore.kernel.org/linux-mm/20240320210716.GH294822@xxxxxxxxxxx/ > > > > > > > > > > > > > > Signed-off-by: Yosry Ahmed <yosryahmed@xxxxxxxxxx> > > > > > > > > > > > > This is an interesting direction to pursue, but I actually thinkg it > > > > > > doesn't go far enough. Either way, I think it needs more data. > > > > > > > > > > > > 1) How frequent are non-zero-same-filled pages? Difficult to > > > > > > generalize, but if you could gather some from your fleet, that > > > > > > would be useful. If you can devise a portable strategy, I'd also be > > > > > > more than happy to gather this on ours (although I think you have > > > > > > more widespread zswap use, whereas we have more disk swap.) > > > > > > > > > > I am trying to collect the data, but there are.. hurdles. It would > > > > > take some time, so I was hoping the data could be collected elsewhere > > > > > if possible. > > > > > > > > > > The idea I had was to hook a BPF program to the entry of > > > > > zswap_fill_page() and create a histogram of the "value" argument. We > > > > > would get more coverage by hooking it to the return of > > > > > zswap_is_page_same_filled() and only updating the histogram if the > > > > > return value is true, as it includes pages in zswap that haven't been > > > > > swapped in. > > > > > > > > > > However, with zswap_is_page_same_filled() the BPF program will run in > > > > > all zswap stores, whereas for zswap_fill_page() it will only run when > > > > > needed. Not sure if this makes a practical difference tbh. > > > > > > > > > > > > > > > > > 2) The fact that we're doing any of this pattern analysis in zswap at > > > > > > all strikes me as a bit misguided. Being efficient about repetitive > > > > > > patterns is squarely in the domain of a compression algorithm. Do > > > > > > we not trust e.g. zstd to handle this properly? > > > > > > > > > > I thought about this briefly, but I didn't follow through. I could try > > > > > to collect some data by swapping out different patterns and observing > > > > > how different compression algorithms react. That would be interesting > > > > > for sure. > > > > > > > > > > > > > > > > > I'm guessing this goes back to inefficient packing from something > > > > > > like zbud, which would waste half a page on one repeating byte. > > > > > > > > > > > > But zsmalloc can do 32 byte objects. It's also a batching slab > > > > > > allocator, where storing a series of small, same-sized objects is > > > > > > quite fast. > > > > > > > > > > > > Add to that the additional branches, the additional kmap, the extra > > > > > > scanning of every single page for patterns - all in the fast path > > > > > > of zswap, when we already know that the vast majority of incoming > > > > > > pages will need to be properly compressed anyway. > > > > > > > > > > > > Maybe it's time to get rid of the special handling entirely? > > > > > > > > > > We would still be wasting some memory (~96 bytes between zswap_entry > > > > > and zsmalloc object), and wasting cycling allocating them. This could > > > > > be made up for by cycles saved by removing the handling. We will be > > > > > saving some branches for sure. I am not worried about kmap as I think > > > > > it's a noop in most cases. > > > > > > > > Yes, true. > > > > > > > > > I am interested to see how much we could save by removing scanning for > > > > > patterns. We may not save much if we abort after reading a few words > > > > > in most cases, but I guess we could also be scanning a considerable > > > > > amount before aborting. On the other hand, we would be reading the > > > > > page contents into cache anyway for compression, so maybe it doesn't > > > > > really matter? > > > > > > > > > > I will try to collect some data about this. I will start by trying to > > > > > find out how the compression algorithms handle same-filled pages. If > > > > > they can compress it efficiently, then I will try to get more data on > > > > > the tradeoff from removing the handling. > > > > > > > > I do wonder if this could be overthinking it, too. > > > > > > > > Double checking the numbers on our fleet, a 96 additional bytes for > > > > each same-filled entry would result in a > > > > > > > > 1) p50 waste of 0.008% of total memory, and a > > > > > > > > 2) p99 waste of 0.06% of total memory. > > > > Right. Assuming the compressors do not surprise us and store > > same-filled pages in an absurd way, it's not worth it in terms of > > memory savings. > > > > > > > > > > And this is without us having even thought about trying to make > > > > zsmalloc more efficient for this particular usecase - which might be > > > > the better point of attack, if we think it's actually worth it. > > > > > > > > So my take is that unless removing it would be outright horrible from > > > > a %sys POV (which seems pretty unlikely), IMO it would be fine to just > > > > delete it entirely with a "not worth the maintenance cost" argument. > > > > > > > > If you turn the argument around, and somebody would submit the code as > > > > it is today, with the numbers being what they are above, I'm not sure > > > > we would even accept it! > > > > > > The context guy is here :) > > > > > > Not arguing for one way or another, but I did find the original patch > > > that introduced same filled page handling: > > > > > > https://github.com/torvalds/linux/commit/a85f878b443f8d2b91ba76f09da21ac0af22e07f > > > > > > https://lore.kernel.org/all/20171018104832epcms5p1b2232e2236258de3d03d1344dde9fce0@epcms5p1/T/#u > > > > Thanks for digging this up. I don't know why I didn't start there :) > > > > Following in your footsteps, and given that zram has the same feature, > > I found the patch that added support for non-zero same-filled pages in > > zram: > > https://lore.kernel.org/all/1483692145-75357-1-git-send-email-zhouxianrong@xxxxxxxxxx/#t > > > > Both of them confirm that most same-filled pages are zero pages, but > > they show a more significant portion of same-filled pages being > > non-zero (17% to 40%). I suspect this will be less in data centers > > compared to consumer apps. > > > > The zswap patch also reports significant performance improvements from > > the same-filled handling, but this is with 17-22% same-filled pages. > > Johannes mentioned around 10% in your data centers, so the performance > > improvement would be less. In the kernel build tests I ran with only > > around 1.5% same-filled pages I observed 1.4% improvements just by > > optimizing them (only zero-filled, skipping allocations). > > > > So I think removing the same-filled pages handling completely may be > > too aggressive, because it doesn't only affect the memory efficiency, > > but also cycles spent when handling those pages. Just avoiding going > > through the allocator and compressor has to account for something :) > > Here is another data point. I tried removing the same-filled handling > code completely with the diff Johannes sent upthread. I saw 1.3% > improvement in the kernel build test, very similar to the improvement > from this patch series. _However_, the kernel build test only produces > ~1.5% zero-filled pages in my runs. More realistic workloads have > significantly higher percentages as demonstrated upthread. > > In other words, the kernel build test (at least in my runs) seems to > be the worst case scenario for same-filled/zero-filled pages. Since > the improvement from removing same-filled handling is quite small in > this case, I suspect there will be no improvement, but possibly a > regression, on real workloads. > > As the zero-filled pages ratio increases: > - The performance with this series will improve. > - The performance with removing same-filled handling completely will > become worse. Sorry, this thread is still really lacking practical perspective. As do the numbers that initially justified the patch. Sure, the stores of same-filled pages are faster. What's the cost of prechecking 90% of the other pages that need compression? Also, this is the swap path we're talking about. There is vmscan, swap slot allocations, page table walks, TLB flushes, zswap tree inserts; then a page fault and everything in reverse. I perf'd zswapping out data that is 10% same-filled and 90% data that always needs compression. It does nothing but madvise(MADV_PAGEOUT), and the zswap_store() stack is already only ~60% of the cycles. Using zsmalloc + zstd, this is the diff between vanilla and my patch: # Baseline Delta Abs Shared Object Symbol # ........ ......... .................... ..................................................... # 4.34% -3.02% [kernel.kallsyms] [k] zswap_store 11.07% +1.41% [kernel.kallsyms] [k] ZSTD_compressBlock_doubleFast 15.55% +0.91% [kernel.kallsyms] [k] FSE_buildCTable_wksp As expected, we have to compress a bit more; on the other hand we're removing the content scan for same-filled for 90% of the pages that don't benefit from it. They almost amortize each other. Let's round it up and the remaining difference is ~1%. It's difficult to make the case that this matters to any real workloads with actual think time in between paging. But let's say you do make the case that zero-filled pages are worth optimizing for. Why is this in zswap? Why not do it in vmscan with a generic zero-swp_entry_t, and avoid the swap backend altogether? No swap slot allocation, no zswap tree, no *IO on disk swap*. However you slice it, I fail to see how this has a place in zswap. It's trying to optimize the slow path of a slow path, at the wrong layer of the reclaim stack.
