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Use a less aggressive query buffer resize policy.

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A user with many connections (10 thousand) on a single Redis server
reports in issue #4983 that sometimes Redis is idle becuase at the same
time many clients need to resize their query buffer according to the old
policy.

It looks like this was created by the fact that we allow the query
buffer to grow without problems to a size up to PROTO_MBULK_BIG_ARG
normally, but when the client is idle we immediately are more strict,
and a query buffer greater than 1024 bytes is already enough to trigger
the resize. So for instance if most of the clients stop at the same time
this issue should be easily triggered.

This behavior actually looks odd, and there should be only a clear limit
after we say, let's look at this query buffer to check if it's time to
resize it. This commit puts the limit at PROTO_MBULK_BIG_ARG, and the
check is performed both if compared to the peak usage the current usage
is too big, or if the client is idle.

Then when the check is performed, to waste just a few kbytes is
considered enough to proceed with the resize. This should fix the issue.
This commit is contained in:
antirez 2018-06-11 17:12:28 +02:00
parent 5bbb00fbb4
commit cec404f099
1 changed files with 7 additions and 6 deletions
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13
src/server.c
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@ -845,13 +845,14 @@ int clientsCronResizeQueryBuffer(client *c) {
/* There are two conditions to resize the query buffer:
* 1) Query buffer is > BIG_ARG and too big for latest peak.
* 2) Client is inactive and the buffer is bigger than 1k. */
if (((querybuf_size > PROTO_MBULK_BIG_ARG) &&
(querybuf_size/(c->querybuf_peak+1)) > 2) ||
(querybuf_size > 1024 && idletime > 2))
* 2) Query buffer is > BIG_ARG and client is idle. */
if (querybuf_size > PROTO_MBULK_BIG_ARG &&
((querybuf_size/(c->querybuf_peak+1)) > 2 ||
idletime > 2))
{
/* Only resize the query buffer if it is actually wasting space. */
if (sdsavail(c->querybuf) > 1024) {
/* Only resize the query buffer if it is actually wasting
* at least a few kbytes. */
if (sdsavail(c->querybuf) > 1024*4) {
c->querybuf = sdsRemoveFreeSpace(c->querybuf);
}
}