RFR: JDK-8337974 - ChannelInputStream::skip can use splice (linux) [v2]

[Markus KARG]

On Sun, 27 Oct 2024 14:25:07 GMT, Markus KARG <duke at openjdk.org> wrote:

>> # Targets
>> 
>> The major target of this issue is to reduce execution time of `ChannelInputStream::skip(long)`. In particular, make `skip(n)` run noticable faster than `read(new byte[n])` on pipes and sockets in the optimal case, but don't run noticable slower in the worst case.
>> 
>> A side target of this issue is to provide unit testing for `ChannelInputStream::skip(long)`. In particular, provide unit testing for files, pipes and sockets.
>> 
>> An appreciated benefit of this issue is reduced resource consumption (in the sense of CPU cycles, Java Heap, power consumption, CO2 footprint, etc.) of `ChannelInputStream::skip(long)`. Albeit, as it is not a target, this was not acitvely monitored.
>> 
>> 
>> # Non-Targets
>> 
>> It is not a target to improve any other methods of the mentioned or any other class. Such changes should go in separate issues.
>> 
>> It is not a target to add any new *public* APIs. The public API shall be kept unchanged. All changes implied by the current improvement shall be purely *internal* to OpenJDK.
>> 
>> It is not a target to improve other source types besides pipes and sockets.
>> 
>> 
>> # Description
>> 
>> What users expect from `InputStream::skip`, besides comfort, is "at least some" measurable benefit over `InputStream::read`. Otherwise skipping instead of reading makes no sense to users.
>> 
>> For files, OpenJDK already applies an effective `seek`-based optimization. For pipes and sockets, benefits were neglectible so far, as `skip` more or less was simply an alias for `read`.
>> 
>> Hence, this issue proposes optimized implementations for `ChannelInputStream::skip` in the pipes and sockets cases.
>> 
>> 
>> # Implementation
>> 
>> The abstract idea behind this issue is to prevent transmission of skipped data into the JVM's on-heap memory in the pipes and socket cases. As a Java application obviously is not interested in skipped data, copying it into the Java heap is a waste of both, time and heap, and induces (albeit neglectible) GC stress without any benefit.
>> 
>> Hence, this pull request changes the implementation of `ChannelInputStream::skip` in the following aspects:
>> 1. On *all* operating systems, for pipe and socket channels, `skip` is implemented in C. While the data *still is* transferred form the source into the OS kernel and from the OS kernel into the JVM's off-heap memory, it is *not* transferred into the JVM's on-heap memory.
>> 2. For *Linux* pipes only, `splice` is used with `/dev/null` as the target. Data is neither transferr...
>
> Markus KARG has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Socket.getInputStream()'s JavaDocs explain behavior of skip for timeout, non-blocking, interrupt

Kindly asking to resume this suspended review.

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PR Comment: https://git.openjdk.org/jdk/pull/20489#issuecomment-2564654972