István Fajth created HDFS-15692:
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Summary: Improve furse_dfs read performace
Key: HDFS-15692
URL: https://issues.apache.org/jira/browse/HDFS-15692
Project: Hadoop HDFS
Issue Type: Improvement
Components: fuse-dfs
Reporter: István Fajth
Currently fuse_dfs uses a prefetch buffer to read from HDFS via libhdfs' pread
method.
The algorithm inside fuse_read.c in short does the following:
if the rdbuffer size is less then the buffer provided
then
reads directly to the buffer
else
grab lock
if the preftch buffer does not have more data
then
fills the prefetch buffer
endif
fills the supplied buffer via memcpy from the prefetch buffer
release lock
endif
It would be nice to have a background thread and double prefetch buffers, so
while one buffer serves the reads coming from the local client, the other can
prefetch the data, with that we can improve the read speed, especially with EC
encoded files.
According to some measurements I did, if I increase the read buffer, there is a
significant change in runtime, with 64MB the runtime is really closer to HDFS
by a large margin. Interestingly 128MB as the buffer size does not perform
well, but 256MB is even more closer to what the dfs client can provide. (16 vs
18 seconds with rep3 files, while in par with ec encoded files dfs vs fuse)
So it seems it is worth to stream continuously a larger chunk of data, at least
with pread, but in case we have a separate fetching thread and double
buffering, we don't even need positioned reads, simply just continuous
streaming of data with read.
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