The current hostcache hashing mechanism apparently
ends up with an very uneven distribution of the
entries into the hostcache hash array.
This was observed by adding a histogram output
function.
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I'm contemplating if it makes sense to just provide the V_tcp_hostcache.hashbase[i].hch_length for each bucket and allow to user to do whatever analysis is wanted. I think the focus should be an admin using the system, not a developer debugging it. So we don't need to to count the entries and provide both information. I guess that is only interesting for us right now...
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Indeed, fair point. removing the (costly) validation against the actual bucket (tailq) length, since the lock around that counter is highly unlikely to have contributed to an issue for hostcache.count.
| sys/netinet/tcp_hostcache.c | ||
|---|---|---|
| 735 | I think you need V_tcp_hostcache.bucket_limit + 1 here. | |
| 745 | The check needs to be <= instead of <. For consistency you might want to get the mutex. | |
| 746 | What using a KASSERT V_tcp_hostcache.hashbase[i].hch_length <= V_tcp_hostcache.bucket_limit and then just update the array. | |
| 754 | You need <= instead of <. | |
| sys/netinet/tcp_hostcache.c | ||
|---|---|---|
| 735 | Indeed. | |
| 745 | I want to avoid the cost of the LOCK/UNLOCK, at the cost of absolute accuracy. Similar to the hostcache.list - which will dump the contents of each hashindex at different times... The idea for this histogram is more to give a sense of the hash usage distribution (which doesn't need to be perfectly accurate). E.g. the jenkins_hash32 offers the possibility to change a salt. Or at some future point, a dynamic resizing (hash array width vs. depth) could be thought about. | |
| 746 | Since this is an advisory output only, I rather hint at a non-catastrophic state (the bucket should no longer grow - but that could be monitored over time, if detected), than stopping the system. If such a invalid state needs to be frozen and analyzed, there are other ways to achieve this already. | |
| sys/netinet/tcp_hostcache.c | ||
|---|---|---|
| 746 | My point is that I consider the occurrence of hch_length > V_tcp_hostcache.bucket_limit a bug in the kernel. So I would put a message on the console, not include it in the sysctl output... | |
| sys/netinet/tcp_hostcache.c | ||
|---|---|---|
| 725 | I guess this needs to be unsigned int, since hch_length is unsigned. | |
| 777 | This part of the condition is always true, since hch_length is an unsigned int. | |
| 782 | I would move the KASSERT up, just before it is decremented and check that it is positive and not exceeding the limit. | |
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Please note that you are not only adding the histogram features anymore, but also added verification code that hch_length is sane. You might to to reflect that in the commit message.