Summary:
This PR extends the improvements in #3282 to also work when using Direct IO.
We see **4.5X performance improvement** in seekrandom benchmark doing long range scans, when using direct reads, on flash.
**Description:**
This change improves the performance of iterators doing long range scans (e.g. big/full index or table scans in MyRocks) by using readahead and prefetching additional data on each disk IO, and storing in a local buffer. This prefetching is automatically enabled on noticing more than 2 IOs for the same table file during iteration. The readahead size starts with 8KB and is exponentially increased on each additional sequential IO, up to a max of 256 KB. This helps in cutting down the number of IOs needed to complete the range scan.
**Implementation Details:**
- Used `FilePrefetchBuffer` as the underlying buffer to store the readahead data. `FilePrefetchBuffer` can now take file_reader, readahead_size and max_readahead_size as input to the constructor, and automatically do readahead.
- `FilePrefetchBuffer::TryReadFromCache` can now call `FilePrefetchBuffer::Prefetch` if readahead is enabled.
- `AlignedBuffer` (which is the underlying store for `FilePrefetchBuffer`) now takes a few additional args in `AlignedBuffer::AllocateNewBuffer` to allow copying data from the old buffer.
- Made sure not to re-read partial chunks of data that were already available in the buffer, from device again.
- Fixed a couple of cases where `AlignedBuffer::cursize_` was not being properly kept up-to-date.
**Constraints:**
- Similar to #3282, this gets currently enabled only when ReadOptions.readahead_size = 0 (which is the default value).
- Since the prefetched data is stored in a temporary buffer allocated on heap, this could increase the memory usage if you have many iterators doing long range scans simultaneously.
- Enabled only for user reads, and disabled for compactions. Compaction reads are controlled by the options `use_direct_io_for_flush_and_compaction` and `compaction_readahead_size`, and the current feature takes precautions not to mess with them.
**Benchmarks:**
I used the same benchmark as used in #3282.
Data fill:
```
TEST_TMPDIR=/data/users/$USER/benchmarks/iter ./db_bench -benchmarks=fillrandom -num=1000000000 -compression_type="none" -level_compaction_dynamic_level_bytes
```
Do a long range scan: Seekrandom with large number of nexts
```
TEST_TMPDIR=/data/users/$USER/benchmarks/iter ./db_bench -benchmarks=seekrandom -use_direct_reads -duration=60 -num=1000000000 -use_existing_db -seek_nexts=10000 -statistics -histogram
```
```
Before:
seekrandom : 37939.906 micros/op 26 ops/sec; 29.2 MB/s (1636 of 1999 found)
With this change:
seekrandom : 8527.720 micros/op 117 ops/sec; 129.7 MB/s (6530 of 7999 found)
```
~4.5X perf improvement. Taken on an average of 3 runs.
Closes https://github.com/facebook/rocksdb/pull/3884
Differential Revision: D8082143
Pulled By: sagar0
fbshipit-source-id: 4d7a8561cbac03478663713df4d31ad2620253bb
Summary:
Provide a block_align option in BlockBasedTableOptions to allow
alignment of SST file data blocks. This will avoid higher
IOPS/throughput load due to < 4KB data blocks spanning 2 4KB pages.
When this option is set to true, the block alignment is set to lower of
block size and 4KB.
Closes https://github.com/facebook/rocksdb/pull/3502
Differential Revision: D7400897
Pulled By: anand1976
fbshipit-source-id: 04cc3bd144e88e3431a4f97604e63ad7a0f06d44
Summary:
FilePrefetchBuffer::Prefetch is currently rounds the offset up which does not fit its new use cases in prefetching index/filter blocks, as it would skips over some the offsets that were requested to be prefetched. This patch rounds down instead.
Fixes#3180
Closes https://github.com/facebook/rocksdb/pull/3413
Differential Revision: D6816392
Pulled By: maysamyabandeh
fbshipit-source-id: 3aaeaf59c55d72b61dacfae6d4a8e65eccb3c553
Summary:
Currently for direct I/O, the large maximum buffer is always allocated. This will be wasteful if users flush the data in much smaller chunks. This diff fix this by changing the behavior of incremental buffer works. When we enlarge buffer, we try to copy the existing data in the buffer to the enlarged buffer, rather than flush the buffer first. This can make sure that no extra I/O is introduced because of buffer enlargement.
Closes https://github.com/facebook/rocksdb/pull/2403
Differential Revision: D5178403
Pulled By: siying
fbshipit-source-id: a8fe1e7304bdb8cab2973340022fe80ff83449fd
Summary:
Remove double buffering on RandomRead on Windows.
With more logic appear in file reader/write Read no longer
obeys forwarding calls to Windows implementation.
Previously direct_io (unbuffered) was only available on Windows
but now is supported as generic.
We remove intermediate buffering on Windows.
Remove random_access_max_buffer_size option which was windows specific.
Non-zero values for that opton introduced unnecessary lock contention.
Remove Env::EnableReadAhead(), Env::ShouldForwardRawRequest() that are
no longer necessary.
Add aligned buffer reads for cases when requested reads exceed read ahead size.
Closes https://github.com/facebook/rocksdb/pull/2105
Differential Revision: D4847770
Pulled By: siying
fbshipit-source-id: 8ab48f8e854ab498a4fd398a6934859792a2788f
Summary:
direct IO reads refactoring
remove unnecessary classes and unified interfaces
tested with db_bench
need more change for options and ON/OFF for different files.
Since disabled is default, it should be fine now
Closes https://github.com/facebook/rocksdb/pull/1636
Differential Revision: D4307189
Pulled By: lightmark
fbshipit-source-id: 6991e22
Summary. A change https://reviews.facebook.net/differential/diff/224721/
Has attempted to move common functionality out of platform dependent
code to a new facility called file_reader_writer.
This includes:
- perf counters
- Buffering
- RateLimiting
However, the change did not attempt to refactor Windows code.
To mitigate, we introduce new quering interfaces such as UseOSBuffer(),
GetRequiredBufferAlignment() and ReaderWriterForward()
for pure forwarding where required.
Introduce WritableFile got a new method Truncate(). This is to communicate
to the file as to how much data it has on close.
- When space is pre-allocated on Linux it is filled with zeros implicitly,
no such thing exist on Windows so we must truncate file on close.
- When operating in unbuffered mode the last page is filled with zeros but we still want to truncate.
Previously, Close() would take care of it but now buffer management is shifted to the wrappers and the file has
no idea about the file true size.
This means that Close() on the wrapper level must always include
Truncate() as well as wrapper __dtor should call Close() and
against double Close().
Move buffered/unbuffered write logic to the wrapper.
Utilize Aligned buffer class.
Adjust tests and implement Truncate() where necessary.
Come up with reasonable defaults for new virtual interfaces.
Forward calls for RandomAccessReadAhead class to avoid double
buffering and locking (double locking in unbuffered mode on WIndows).
Sagar Vemuri