Thomas Jackson created TS-4331:
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Summary: Hostdb consistency problems due to MultiCache
Key: TS-4331
URL: https://issues.apache.org/jira/browse/TS-4331
Project: Traffic Server
Issue Type: Bug
Reporter: Thomas Jackson
This ticket is for the correct long term fix to TS-4207
pulled from a comment, which wraps up the issue
{quote}
Leif Hedstrom I have spent a decent amount of time on this while I was OOO on
vacation the last couple of weeks. It seems that the root cause of this issue
has always existed, and that the addition of always doing hostname storing
(https://github.com/apache/trafficserver/commit/0e703e1e) we are just causing
the issue to happen all the time.
To understand the issue I'll give a little background in how hostdb is
currently working. Basically hostdb is just a wrapper around this templated
struct called MultiCache. MultiCache is "multi" not because it is templated,
but because it has two types of storage (static-- blocks and dynamic-- alloc).
The static side of the cache can hold N HostDBInfo structs (the results of DNS
queries). The dynamic side is used to store the round robin records and various
strings associated with the record. The size of this dynamic space is defined
as (N x [estimated_heap_bytes_per_entry. The basic problem we are running into
is that we are putting too much preassure on the dynamic heap-- such that the
heap is getting re-used while people still have references to items in that
space.
So, I've actually been working on re-writing MultiCache to allocate the entire
required block at once (so we don't have this problem where the parent exists
but not the children), but I'm not certain if we want such a change to go into
the 6.x branch (I'm willing to discuss if we want). If we aren't comfortable
with such a large change I suggest just accounting for the hostname size in the
estimated_heap_bytes_per_entry as a stopgap solution. The maximum allowable
size is 253 (so 254 with null terminator), but we could pick a smaller number
(~120 or so seems to be more reasonable). Alternatively you can increase the
number of records in hostdb (and the size accordingly) to increase the dynamic
heap size.
TLDR; almost done with the long term solution, but I'm not sure if we want to
merge that into 6.x-- alternatively we can do a simple workaround in 6.x
(https://github.com/apache/trafficserver/pull/553)
{quote}
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