mmap is used for faster reads and as you guessed right you might see read performance degradation. If you are seeing high memory usage after repairs due to mmaped files, the only way to reduce the memory usage is to trigger some other process which requires memory. *mmapped* files use buffer/cache memory which gets released as soon as some other process requests memory from the kernel. Kernel does not want to waste its effort until some request for resource comes in.
On Tue, Aug 3, 2021 at 4:42 AM Amandeep Srivastava < amandeep.srivastava1...@gmail.com> wrote: > Can anyone please help with the above questions? To summarise: > > 1) What is the impact of using mmap only for indices besides a degradation > in read performance? > 2) Why does the off heap consumed during Cassandra full repair remains > occupied 12+ hours after the repair completion and is there a > manual/configuration driven way to clear that earlier? > > Thanks, > Aman > > On Thu, 29 Jul, 2021, 6:47 pm Amandeep Srivastava, < > amandeep.srivastava1...@gmail.com> wrote: > >> Hi Erick, >> >> Limiting mmap to index only seems to have resolved the issue. The max ram >> usage remained at 60% this time. Could you please point me to the >> limitations for setting this param? - For starters, I can see read >> performance getting reduced up to 30% (CASSANDRA-8464 >> <https://issues.apache.org/jira/browse/CASSANDRA-8464>) >> >> Also if you could please shed light on extended questions in my earlier >> email. >> >> Thanks a lot. >> >> Regards, >> Aman >> >> On Thu, Jul 29, 2021 at 12:52 PM Amandeep Srivastava < >> amandeep.srivastava1...@gmail.com> wrote: >> >>> Thanks, Bowen, don't think that's an issue - but yes I can try upgrading >>> to 3.11.5 and limit the merkle tree size to bring down the memory >>> utilization. >>> >>> Thanks, Erick, let me try that. >>> >>> Can someone please share documentation relating to internal functioning >>> of full repairs - if there exists one? Wanted to understand the role of the >>> heap and off-heap memory separately during the process. >>> >>> Also, for my case, once the nodes reach the 95% memory usage, it stays >>> there for almost 10-12 hours after the repair is complete, before falling >>> back to 65%. Any pointers on what might be consuming off-heap for so long >>> and can something be done to clear it earlier? >>> >>> Thanks, >>> Aman >>> >>> >>> >> >> -- >> Regards, >> Aman >> >
