From e6918187568dbd01842d8d1d2c808ce16a894239 Mon Sep 17 00:00:00 2001 From: Daniel Baumann Date: Sun, 21 Apr 2024 13:54:28 +0200 Subject: Adding upstream version 18.2.2. Signed-off-by: Daniel Baumann --- .../docs/_posts/2017-08-24-pinnableslice.markdown | 37 ++++++++++++++++++++++ 1 file changed, 37 insertions(+) create mode 100644 src/rocksdb/docs/_posts/2017-08-24-pinnableslice.markdown (limited to 'src/rocksdb/docs/_posts/2017-08-24-pinnableslice.markdown') diff --git a/src/rocksdb/docs/_posts/2017-08-24-pinnableslice.markdown b/src/rocksdb/docs/_posts/2017-08-24-pinnableslice.markdown new file mode 100644 index 000000000..06e0bcb2f --- /dev/null +++ b/src/rocksdb/docs/_posts/2017-08-24-pinnableslice.markdown @@ -0,0 +1,37 @@ +--- +title: PinnableSlice; less memcpy with point lookups +layout: post +author: maysamyabandeh +category: blog +--- + +The classic API for [DB::Get](https://github.com/facebook/rocksdb/blob/9e583711144f580390ce21a49a8ceacca338fcd5/include/rocksdb/db.h#L310) receives a std::string as argument to which it will copy the value. The memcpy overhead could be non-trivial when the value is large. The [new API](https://github.com/facebook/rocksdb/blob/9e583711144f580390ce21a49a8ceacca338fcd5/include/rocksdb/db.h#L322) receives a PinnableSlice instead, which avoids memcpy in most of the cases. + +### What is PinnableSlice? + +Similarly to Slice, PinnableSlice refers to some in-memory data so it does not incur the memcpy cost. To ensure that the data will not be erased while it is being processed by the user, PinnableSlice, as its name suggests, has the data pinned in memory. The pinned data are released when PinnableSlice object is destructed or when ::Reset is invoked explicitly on it. + +### How good is it? + +Here are the improvements in throughput for an [in-memory benchmark](https://github.com/facebook/rocksdb/pull/1756#issuecomment-286201693): +* value 1k byte: 14% +* value 10k byte: 34% + +### Any limitations? + +PinnableSlice tries to avoid memcpy as much as possible. The primary gain is when reading large values from the block cache. There are however cases that it would still have to copy the data into its internal buffer. The reason is mainly the complexity of implementation and if there is enough motivation on the application side. the scope of PinnableSlice could be extended to such cases too. These include: +* Merged values +* Reads from memtables + +### How to use it? + +```cpp +PinnableSlice pinnable_val; +while (!stopped) { + auto s = db->Get(opt, cf, key, &pinnable_val); + // ... use it + pinnable_val.Reset(); // then release it immediately +} +``` + +You can also [initialize the internal buffer](https://github.com/facebook/rocksdb/blob/9e583711144f580390ce21a49a8ceacca338fcd5/include/rocksdb/db.h#L314) of PinnableSlice by passing your own string in the constructor. [simple_example.cc](https://github.com/facebook/rocksdb/blob/main/examples/simple_example.cc) demonstrates that with more examples. -- cgit v1.2.3