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authorWladimir J. van der Laan <laanwj@protonmail.com>2019-09-30 09:47:16 +0200
committerWladimir J. van der Laan <laanwj@protonmail.com>2019-09-30 09:47:28 +0200
commit6b9405e3191206f1cda626423aab6a453980ab05 (patch)
treef7d39cb037d4fdd6ae27e6d9c9b916d511ee315e
parent26a74370455cc03c4e7be73728775265ed227a9f (diff)
parent67d99900b0d770038c9c5708553143137b124a6c (diff)
Merge #16957: 9% less memory: make SaltedOutpointHasher noexcept
67d99900b0d770038c9c5708553143137b124a6c make SaltedOutpointHasher noexcept (Martin Ankerl) Pull request description: If the hash is not `noexcept`, `unorderd_map` has to assume that it can throw an exception. Thus when rehashing care needs to be taken. libstdc++ solves this by simply caching the hash value, which increases memory of each node by 8 bytes. Adding `noexcept` prevents this caching. In my experiments with `-reindex-chainstate -stopatheight=594000`, memory usage (maximum resident set size) has decreased by 9.4% while runtime has increased by 1.6% due to additional hashing. Additionally, memusage::DynamicUsage() is now more accurate and does not underestimate. | | runtime h:mm:ss | max RSS kbyte | |---------------------------------------|-----------------|--------------| | master | 4:13:59 | 7696728 | | 2019-09-SaltedOutpointHasher-noexcept | 4:18:11 | 6971412 | | change | +1.65% | -9,42% | Comparison of progress masters vs. 2019-09-SaltedOutpointHasher-noexcept ![out](https://user-images.githubusercontent.com/14386/65541887-69424e00-df0e-11e9-8644-b3a068ed8c3f.png) ACKs for top commit: jamesob: Tested ACK https://github.com/bitcoin/bitcoin/pull/16957/commits/67d99900b0d770038c9c5708553143137b124a6c Tree-SHA512: 9c44e3cca993b5a564dd61ebd2926b9c4a238609ea4d283514c018236f977d935e35a384dd4696486fd3d78781dd2ba190bb72596e20a5e931042fa465872a0b
-rw-r--r--src/coins.h10
1 files changed, 9 insertions, 1 deletions
diff --git a/src/coins.h b/src/coins.h
index dca1beabb6..d8135e0d9a 100644
--- a/src/coins.h
+++ b/src/coins.h
@@ -95,8 +95,16 @@ public:
* This *must* return size_t. With Boost 1.46 on 32-bit systems the
* unordered_map will behave unpredictably if the custom hasher returns a
* uint64_t, resulting in failures when syncing the chain (#4634).
+ *
+ * Having the hash noexcept allows libstdc++'s unordered_map to recalculate
+ * the hash during rehash, so it does not have to cache the value. This
+ * reduces node's memory by sizeof(size_t). The required recalculation has
+ * a slight performance penalty (around 1.6%), but this is compensated by
+ * memory savings of about 9% which allow for a larger dbcache setting.
+ *
+ * @see https://gcc.gnu.org/onlinedocs/gcc-9.2.0/libstdc++/manual/manual/unordered_associative.html
*/
- size_t operator()(const COutPoint& id) const {
+ size_t operator()(const COutPoint& id) const noexcept {
return SipHashUint256Extra(k0, k1, id.hash, id.n);
}
};