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 24 2018-06-10T02:48:08  <gmaxwell> sipa: did you look at their implementation? https://github.com/armfazh/flo-shani-aesni/blob/master/sha256/flo-shani.c
 25 2018-06-10T02:52:48  <sipa> gmaxwell: yes, just interleaving
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 74 2018-06-10T09:08:21  <provoostenator> sipa: thanks for the extra context. Maintaining a large dbcache is mainly useful during IBD, so can't the problem of reorgs be avoided by only doing the optimization for very deep blocks?
 75 2018-06-10T09:10:43  <provoostenator> And just in case, if during IBD an alternative set of headers is found that would trigger a deep reorg, you'd flush the cache and turn off the optimization, before switching to that new branch.
 76 2018-06-10T09:14:31  <provoostenator> Right now it seems that 500 MB < dbcache < 7000 MB is a performance dead zone. Though I can try tweaking #11658 to see where the diminishing returns are.
 77 2018-06-10T09:14:33  <gribble> https://github.com/bitcoin/bitcoin/issues/11658 | During IBD, when doing pruning, prune 10% extra to avoid pruning again soon after by luke-jr · Pull Request #11658 · bitcoin/bitcoin · GitHub
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135 2018-06-10T15:58:17  <provoostenator> Do I understand correctly that the only way for a coin cache entry to be dirty, is if the UTXO existed before the last flush and was spent since then? Would it be worth trying to bypass the cache in those cases and update the disk when spending a UTXO that's not in the cache?
136 2018-06-10T16:01:45  <provoostenator> I wonder if OS's make any effort to optimize a write to the same physical place on disk that you just read from.
137 2018-06-10T16:02:26  <sipa> provoostenator: it can be dirty because it's created after the last flush, or spent after the last flush while ot was created before
138 2018-06-10T16:03:27  <sipa> and of course we can bypass the cache... if we don't care about the performance it offers
139 2018-06-10T16:04:38  <sipa> provoostenator: i guess we could only do the background flushing during IBD, but that's still very scary
140 2018-06-10T16:07:24  <provoostenator> "dirty because it's created after the last flush" - how does that work? I thought they always get the FRESH flag in that case.
141 2018-06-10T16:08:03  <provoostenator> (I meant DIRTY flag, not dirty in general db terminology)
142 2018-06-10T16:09:19  <provoostenator> Of course I do care about the performance impact of such a change. My working theory is that too many DITRY entries slows things down to a state that's worse than a smaller cache. So perhaps preventing accumulation of DIRTY entries would prevent that.
143 2018-06-10T16:10:22  <provoostenator> (my "aggresive" pruning branch is much slower than master, despite the cache growing much bigger)
144 2018-06-10T16:11:33  <provoostenator> I'm currently running IBD from block 320,000 - 480,000 on my iMac several times with decreasing dbcache (and once from genesis without interrupting) to see what happens.
145 2018-06-10T16:18:29  <provoostenator> My hypothesis, based on what I've seen so far, is that when running from genesis to with "infinite" cache, going from 320K to 480K will be fastest. Followed by starting at 320K with infinite cache. A 3 GB cache will be slower, but a 500 MB cache will _faster_ than a 3 GB cache. Possibly regardless of pruning.
146 2018-06-10T16:19:34  <gmaxwell> I think that would be very surprising.
147 2018-06-10T16:20:00  <provoostenator> Indeed
148 2018-06-10T16:24:12  <bitcoin-git> [bitcoin] ken2812221 opened pull request #13426: [WIP, bugfix] Add u8path and u8string to boost to fix #13103 (master...u8path_u8string) https://github.com/bitcoin/bitcoin/pull/13426
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150 2018-06-10T16:31:01  <sipa> provoostenator: FRESH implies DIRTY
151 2018-06-10T16:31:46  <sipa> provoostenator: too many dirty entries slows things down... there may be a memory locality effect from just having many entries, but i don't see any way how dirtyness can impact that
152 2018-06-10T16:32:02  <provoostenator> sipa: ah I see, so I should have said "DIRTY but not FRESH"
153 2018-06-10T16:33:20  <provoostenator> Is there any sorting going on when entries are added?
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155 2018-06-10T16:34:10  <sipa> no
156 2018-06-10T16:34:14  <sipa> it's a hash table
157 2018-06-10T16:34:41  <sipa> provoostenator: i meant to say "about too many dirty entries slowing things doen"
158 2018-06-10T16:34:49  <sipa> i don't believe that can be the case
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160 2018-06-10T16:39:45  <sipa> provoostenator: the time to flush itself may be proportional or worse to the number of dirty entries, though
161 2018-06-10T16:42:17  <provoostenator> From what I saw on my AWS nodes, the pruning (which usually coincided with a cache flash) took just minutes and happened just a dozen or so times, on a IBD measured in days.
162 2018-06-10T16:43:18  <sipa> right
163 2018-06-10T16:43:25  <sipa> that seems expected
164 2018-06-10T16:43:44  <provoostenator> So if an entry is not found int he cache, it starts walking through the disk looking for it? But there's no reason to assume that would be slower than without cache.
165 2018-06-10T16:44:20  <sipa> of course disk will be slower than cache
166 2018-06-10T16:44:57  <sipa> is it possible you're running into swap space?
167 2018-06-10T16:45:27  <provoostenator> Amazon Ubuntu images don't have swap on by default, so I don't think so, but I already deleted those machine.
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169 2018-06-10T16:49:14  <provoostenator> At least I can rule that out in this current experiment, since I have 48 GB RAM
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171 2018-06-10T16:52:25  <provoostenator> When there's a cache, every time it calls CCoinsViewCache::FetchCoin it walks through the memory cache and if nothing is found walks through the disk cache. So there's potentially some duplicate effort, maybe that becomes a problem?
172 2018-06-10T16:53:02  <provoostenator> Oh no, because it's a hash table, it's not walking, it just fetches it.
173 2018-06-10T16:53:51  <provoostenator> The term "iterator" confused me there.
174 2018-06-10T16:54:05  <sipa> yes
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176 2018-06-10T16:54:32  <sipa> and on disk, it just fetches from leveldb, which has indexes and other structure to guide the search - it's isn't really iterating either
177 2018-06-10T16:56:19  <provoostenator> If a big cache causes a slowdown compared to a small cache, it has to be the in-memory stuff I would guess.
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180 2018-06-10T17:04:38  <sipa> how long does flushing take?
181 2018-06-10T17:04:52  <sipa> it can be minutes even on high end systems for multi-gb caches
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183 2018-06-10T17:07:29  <provoostenator> Minutes as far as I know, let me upload the logs...
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186 2018-06-10T17:17:48  <provoostenator> https://ufile.io/tlvv3 (prune3000_sjors.log was the slowest, I gave up after 5 days)
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188 2018-06-10T17:20:44  <provoostenator> TIL about OnionShare, so here you go: http://4nzykwc37ncqcwhp.onion/recall-shiftless
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260 2018-06-10T23:35:09  <sipa> gmaxwell: i win
261 2018-06-10T23:35:48  <sipa> intel's SSE4 sha256 code, transliterated to sse4 intrinsics... is 8% faster than the asm version
262 2018-06-10T23:35:57  <sipa> (on a Ryzen system)
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267 2018-06-10T23:49:30  <sipa> on i7 the intrinsics version is slightly slower (0.7% slower for long hashes, 1.5% slower for double-SHA256, 4$ slower for 32-byte hashes)
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