Tobi Vollebregt created HBASE-12632:
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Summary: ThrottledInputStream/ExportSnapshot does not throttle
Key: HBASE-12632
URL: https://issues.apache.org/jira/browse/HBASE-12632
Project: HBase
Issue Type: Bug
Components: mapreduce
Affects Versions: 0.99.2
Reporter: Tobi Vollebregt
I just transferred a ton of data using ExportSnapshot with bandwidth throttling
from one Hadoop cluster to another Hadoop cluster, and discovered that
ThrottledInputStream does not limit bandwidth.
The problem is that ThrottledInputStream sleeps once, for a fixed time (50 ms),
at the start of each read call, disregarding the actual amount of data read.
ExportSnapshot defaults to a buffer size as big as the block size of the
outputFs:
{code:java}
// Use the default block size of the outputFs if bigger
int defaultBlockSize = Math.max((int) outputFs.getDefaultBlockSize(),
BUFFER_SIZE);
bufferSize = conf.getInt(CONF_BUFFER_SIZE, defaultBlockSize);
LOG.info("Using bufferSize=" + StringUtils.humanReadableInt(bufferSize));
{code}
In my case, this was 256MB.
Hence, the ExportSnapshot mapper will attempt to read up to 256 MB at a time,
each time sleeping only 50ms. Thus, in the worst case where each call to read
fills the 256 MB buffer in negligible time, the ThrottledInputStream cannot
reduce the bandwidth to under (256 MB) / (5 ms) = 5 GB/s.
Even in a more realistic case where read returns about 1 MB per call, it still
cannot throttle the bandwidth to under 20 MB/s.
The issue is exacerbated by the fact that you need to set a low limit because
the total bandwidth per host depends on the number of mapper slots as well.
A simple solution would change the if in throttle to a while, so that it keeps
sleeping for 50 ms until the rate is finally low enough:
{code:java}
private void throttle() throws IOException {
while (getBytesPerSec() > maxBytesPerSec) {
try {
Thread.sleep(SLEEP_DURATION_MS);
totalSleepTime += SLEEP_DURATION_MS;
} catch (InterruptedException e) {
throw new IOException("Thread aborted", e);
}
}
}
{code}
This issue affects the ThrottledInputStream in hadoop as well.
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