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Posted to hdfs-dev@hadoop.apache.org by Dave Marion <dl...@apache.org> on 2023/02/17 13:02:51 UTC
Re: [jira] [Created] (HDFS-16864) HDFS advisory caching should drop cache behind block when block closed
Does anyone have some time to review this 4 line change? Thanks,
Dave
On 2022/12/07 19:06:00 "Dave Marion (Jira)" wrote:
> Dave Marion created HDFS-16864:
> ----------------------------------
>
> Summary: HDFS advisory caching should drop cache behind block when block closed
> Key: HDFS-16864
> URL: https://issues.apache.org/jira/browse/HDFS-16864
> Project: Hadoop HDFS
> Issue Type: Improvement
> Components: hdfs
> Affects Versions: 3.3.4
> Reporter: Dave Marion
>
>
> One of the comments in HDFS-4817 describes the behavior in BlockReceiver.manageWriterOsCache:
>
> "The general idea is that there isn't much point in calling {{sync_file_pages}} twice on the same offsets, since the sync process has presumably already begun. On the other hand, calling {{fadvise(FADV_DONTNEED)}} again and again will tend to purge more and more bytes from the cache. The reason is because dirty pages (those containing un-written-out-data) cannot be purged using {{{}FADV_DONTNEED{}}}. And we can't know exactly when the pages we wrote will be flushed to disk. But we do know that calling {{FADV_DONTNEED}} on very recently written bytes is a waste, since they will almost certainly not have been written out to disk. That is why it purges between 0 and {{{}lastCacheManagementOffset - CACHE_WINDOW_SIZE{}}}, rather than simply 0 to pos."
>
> Looking at the code, I'm wondering if at least the last 8MB (size of CACHE_WINDOW_SIZE) of a block might be left without an associated FADVISE_DONT_NEED call. We're having a [discussion|https://the-asf.slack.com/archives/CERNB8NDC/p1669399302264189] in #accumulo about the file caching feature and I found some interesting [results|https://gist.github.com/dlmarion/1835f387b0fa8fb9dbf849a0c87b6d04] in a test that we wrote. Specifically, that for a multi-block file using setDropBehind with either hsync or CreateFlag.SYNC_BLOCK, parts of each block remained in the cache instead of parts of the last block.
>
> I'm wondering if there is a reason not to call fadvise(FADV_DONTNEED) on the entire block in close [here|https://github.com/apache/hadoop/blob/trunk/hadoop-hdfs-project/hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/BlockReceiver.java#L371] when dropCacheBehindWrites is true.
>
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