Stefan Roesch <shr@xxxxxxxxxxxx> writes: > David Hildenbrand <david@xxxxxxxxxx> writes: > >>>>> Obviously we could spend months analysing which exact allocations are >>>>> identical, and then more months or years reworking the architecture to >>>>> deduplicate them by hand and in userspace. But this isn't practical, >>>>> and KSM is specifically for cases where this isn't practical. >>>>> Based on your request in the previous thread, we investigated whether >>>>> the boost was coming from the unintended side effects of KSM splitting >>>>> THPs. This wasn't the case. >>>>> If you have other theories on how the results could be bogus, we'd be >>>>> happy to investigate those as well. But you have to let us know what >>>>> you're looking for. >>>>> >>>> >>>> Maybe I'm bad at making such requests but >>>> >>>> "Stefan, can you do me a favor and investigate which pages we end up >>>> deduplicating -- especially if it's mostly only the zeropage and if it's >>>> still that significant when disabling THP?" >>>> >>>> "In any case, it would be nice to get a feeling for how much variety in >>>> these 20% of deduplicated pages are. " >>>> >>>> is pretty clear to me. And shouldn't take months. >>>> >> >> Just to clarify: the details I requested are not meant to decide whether to >> reject the patch set (I understand that it can be beneficial to have); I >> primarily want to understand if we're really dealing with a workload where KSM >> is able to deduplicate pages that are non-trivial, to maybe figure out if there >> are other workloads that could similarly benefit -- or if we could optimize KSM >> for these specific cases or avoid the memory deduplication altogether. >> >> In contrast to e.g.: >> >> 1) THP resulted in many zeropages we end up deduplicating again. The THP >> placement was unfortunate. >> >> 2) Unoptimized memory allocators that leave many identical pages mapped >> after freeing up memory (e.g., zeroed pages, pages all filled with >> poison values) instead of e.g., using MADV_DONTNEED to free up that >> memory. >> >> > > I repeated an experiment with and without KSM. In terms of THP there is > no huge difference between the two. On a 64GB main memory machine I see > between 100 - 400MB in AnonHugePages. > >>> /sys/kernel/mm/ksm/pages_shared is over 10000 when we run this on an >>> Instagram workload. The workload consists of 36 processes plus a few >>> sidecar processes. >> >> Thanks! To which value is /sys/kernel/mm/ksm/max_page_sharing set in that >> environment? >> > > It's set to the standard value of 256. > > In the meantime I have run experiments with different settings for > pages_to_scan. With the default value of 100, we only get a relatively > small benefit of KSM. If I increase the value to for instance to 2000 or > 3000 the savings are substantial. (The workload is memory bound, not > CPU bound). > > Here are some stats for setting pages_to_scan to 3000: > > full_scans: 560 > general_profit: 20620539008 > max_page_sharing: 256 > merge_across_nodes: 1 > pages_shared: 125446 > pages_sharing: 5259506 > pages_to_scan: 3000 > pages_unshared: 1897537 > pages_volatile: 12389223 > run: 1 > sleep_millisecs: 20 > stable_node_chains: 176 > stable_node_chains_prune_millisecs: 2000 > stable_node_dups: 2604 > use_zero_pages: 0 > zero_pages_sharing: 0 > > >> What would be interesting is pages_shared after max_page_sharing was set to a >> very high number such that pages_shared does not include duplicates. Then >> pages_shared actually expresses how many different pages we deduplicate. No need >> to run without THP in that case. >> > > Thats on my list for the next set of experiments. > In the new experiment I increased the max_page_sharing value to 16384. This reduced the number of stable_node_dups considerably (its around 3% of the previous value). However pages_sharing is still very high for this workload. full_scans: 138 general_profit: 24442268608 max_page_sharing: 16384 merge_across_nodes: 1 pages_shared: 144590 pages_sharing: 6230983 pages_to_scan: 3000 pages_unshared: 2120307 pages_volatile: 14590780 run: 1 sleep_millisecs: 20 stable_node_chains: 23 stable_node_chains_prune_millisecs: 2000 stable_node_dups: 78 use_zero_pages: 0 zero_pages_sharing: 0 >> Similarly, enabling "use_zero_pages" could highlight if your workload ends up >> deduplciating a lot of zeropages. But maxing out max_page_sharing would be >> sufficient to understand what's happening. >> >> > > I already run experiments with use_zero_pages, but they didn't make a > difference. I'll repeat the experiment with a higher pages_to_scan > value. > >>> Each of these individual processes has around 500MB in KSM pages. >>> >> >> That's really a lot, thanks. >> >>> Also to give some idea for individual VMA's >>> 7ef5d5600000-7ef5e5600000 rw-p 00000000 00:00 0 (Size: 262144 KB, KSM: >>> 73160 KB) >>> >> >> I'll have a look at the patches today.
