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https://issues.apache.org/jira/browse/CASSANDRA-18464?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17736253#comment-17736253
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Ariel Weisberg commented on CASSANDRA-18464:
--------------------------------------------
So short patch :-)
I am not married at all to using MMAP for writing.
For direct IO I think we definitely want to pool two of the commit log buffers,
and then dynamically allocate the rest if writing out commit log segments falls
behind.
I am not sure this is crash safe, I can't remember if extending the length of a
file with direct IO also updates the metadata of the file. Would need to do
some googling. See
https://ext4.wiki.kernel.org/index.php/Clarifying_Direct_IO%27s_Semantics
"Allocating Writes".
Is the issue this is addressing contention around page cache data structures
either when touching each 4k page as we append, or a contention around msync,
or both? It certainly seems like a relatively straightforward way to address it
vs. trying to work around performance issues with msync.
Can you factor out the rest of Cassandra and run commit log stress and
demonstrate the speed difference between the two implementations on NVMe? Here
are some changes I made to help with testing that https://pastebin.com/JXJv0tSd
> Enable Direct I/O For CommitLog Files
> -------------------------------------
>
> Key: CASSANDRA-18464
> URL: https://issues.apache.org/jira/browse/CASSANDRA-18464
> Project: Cassandra
> Issue Type: New Feature
> Components: Local/Commit Log
> Reporter: Josh McKenzie
> Assignee: Amit Pawar
> Priority: Normal
> Fix For: 5.x
>
> Attachments: EnableDirectIOForCommitLogUsingNativeAPI.patch,
> UseDirectIOFeatureForCommitLogFiles.patch
>
>
> Relocating from [dev@ email
> thread.|https://lists.apache.org/thread/j6ny17q2rhkp7jxvwxm69dd6v1dozjrg]
>
> I shared my investigation about Commitlog I/O issue on large core count
> system in my previous email dated July-22 and link to the thread is given
> below.
> [https://lists.apache.org/thread/xc5ocog2qz2v2gnj4xlw5hbthfqytx2n]
> Basically, two solutions looked possible to improve the CommitLog I/O.
> # Multi-threaded syncing
> # Using Direct-IO through JNA
> I worked on 2nd option considering the following benefit compared to the
> first one
> # Direct I/O read/write throughput is very high compared to non-Direct I/O.
> Learnt through FIO benchmarking.
> # Reduces kernel file cache uses which in-turn reduces kernel I/O activity
> for Commitlog files only.
> # Overall CPU usage reduced for flush activity. JVisualvm shows CPU usage <
> 30% for Commitlog syncer thread with Direct I/O feature
> # Direct I/O implementation is easier compared to multi-threaded
> As per the community suggestion, less in code complex is good to have. Direct
> I/O enablement looked promising but there was one issue.
> Java version 8 does not have native support to enable Direct I/O. So, JNA
> library usage is must. The same implementation should also work across other
> versions of Java (like 11 and beyond).
> I have completed Direct I/O implementation and summary of the attached patch
> changes are given below.
> # This implementation is not using Java file channels and file is opened
> through JNA to use Direct I/O feature.
> # New Segment are defined named “DirectIOSegment” for Direct I/O and
> “NonDirectIOSegment” for non-direct I/O (NonDirectIOSegment is test purpose
> only).
> # JNA write call is used to flush the changes.
> # New helper functions are defined in NativeLibrary.java and platform
> specific file. Currently tested on Linux only.
> # Patch allows user to configure optimum block size and alignment if
> default values are not OK for CommitLog disk.
> # Following configuration options are provided in Cassandra.yaml file
> a. use_jna_for_commitlog_io : to use jna feature
> b. use_direct_io_for_commitlog : to use Direct I/O feature.
> c. direct_io_minimum_block_alignment: 512 (default)
> d. nvme_disk_block_size: 32MiB (default and can be changed as per the
> required size)
> Test matrix is complex so CommitLog related testcases and TPCx-IOT benchmark
> was tested. It works with both Java 8 and 11 versions. Compressed and
> Encrypted based segments are not supported yet and it can be enabled later
> based on the Community feedback.
> Following improvement are seen with Direct I/O enablement.
> # 32 cores >= ~15%
> # 64 cores >= ~80%
> Also, another observation would like to share here. Reading Commitlog files
> with Direct I/O might help in reducing node bring-up time after the node
> crash.
> Tested with commit ID: 91f6a9aca8d3c22a03e68aa901a0b154d960ab07
> The attached patch enables Direct I/O feature for Commitlog files. Please
> check and share your feedback.
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