@quentin389 hi, guy. used_memory_rss and mem_fragmentation_ratio have nothing to do with swap.
because, used_memory_rss means how many page redis uses. and fragmentation ratio is below:

mem_fragmentation_ratio = used_memory_rss / zmalloc_used_memory

and in top, your system is using 8348k swap memory

OK, to make myself more clear:

1. Redis documentation @ http://redis.io/commands/info says:
   Ideally, the used_memory_rss value should be only slightly higher than used_memory. When rss >> used, a large difference means there is memory fragmentation (internal or external), which can be evaluated by checking mem_fragmentation_ratio. When used >> rss, it means part of Redis memory has been swapped off by the operating system: expect some significant latencies.

2. So it says that when my fragmentation ratio is < 1 then the serwer is swapping. I have a ratio of 0.9 but swap is not used. Perhaps it's a simple documentation problem and this situation is completely normal. Or maybe it's a redis / system error. I don't know, that's why I have submitted this issue.

3. Furthermore:

• both top and redis report that 'rss' memory usage is 6.5 GB
• redis says it takes 7.2 GB of memory in total. That's 0.7 GB more than 'rss'.
• swap has 8 MB of used memory + 140 MB as buffer + there is a 130 MB buffer in normal memory = even all those things do not go near the 0.7 GB that is not shown as 'rss'
• the system reports a total of 7 GB memory used
• top says that redis has 16 GB virtual memory... err? The whole virtual server has only 8 GB.

4. I don't have any knowledge about how Linux manages / shows memory usage. That's perhaps the reason why I find it really strange that the redis process is suddenly missing 0.7 GB memory, which is nowhere to be found. It's even stranger considering that the documentation claims that it is swapped. Which is clearly not true.

So if there is some obvious reason this happens and it's not an error than I'd appreciate an explanation. I also think that in that situation the documentation should be updated to say that when used >> rss it may mean that we are swapping.
If that's not the case, then perhaps it's some error that requires further investigation.

Hello, in your case used physical memory appears to be smaller than virtual memory but you report no swapped pages. It is likely that in your data set there are many blank pages (all filled with zero) so multiple pages are actually mapped to the same zero page.

Anyway this is never going to be an issue, the worst condition may be an error in the way Redis reports memory, but will have no effects in the stability and functionality of the server.

Thanks for reporting, closing because it is not a bug but just an effect of OS memory management.

If it's an error in Redis memory reporting, woudn't it have a negative effect of capping the used memory too early?

eg. Redis could use this 0.7 GB for additional keys, but it doesn't because used_memory is already equal to maxmemory

@quentin389 I don't think it is possible that in the equation rss/virtual the problem can be in the virtual side, that is the one used by Redis internals, since this is just a counter that is incremented and decremented, so we should be safe. Anyway I don't think it is a problem with memory reporting at all, just that RSS is influenced by many things, including shared and zero-ed pages.

OK, thanks for the explanation :)

No prob at all! Cheers.