> > Oh, well ok that explains why I'm not seeing a flush at 750MB. Sorry, I > was going by the documentation. It claims that the property is around in > 2.0.
But something else is wrong, as Cassandra will crash if you supply an invalid property, implying it's not sourcing the config file you're using. I'm afraid I don't have the context for why it was removed, but it happened as part of the 2.0 release. > On 4 June 2014 13:59, Jack Krupansky <j...@basetechnology.com> wrote: > Yeah, it is in the doc: > > http://www.datastax.com/documentation/cassandra/2.0/cassandra/configuration/configCassandra_yaml_r.html > > And I don’t find a Jira issue mentioning it being removed, so... what’s > the full story there?! > > -- Jack Krupansky > > *From:* Idrén, Johan <johan.id...@dice.se> > *Sent:* Wednesday, June 4, 2014 8:26 AM > *To:* user@cassandra.apache.org > *Subject:* RE: memtable mem usage off by 10? > > > Oh, well ok that explains why I'm not seeing a flush at 750MB. Sorry, I > was going by the documentation. It claims that the property is around in > 2.0. > > > > If we skip that, part of my reply still makes sense: > > > > Having memtable_total_size_in_mb set to 20480, memtables are flushed at a > reported value of ~2GB. > > > > With a constant overhead of ~10x, as suggested, this would mean that it > used 20GB, which is 2x the size of the heap. > > > > That shouldn't work. According to the OS, cassandra doesn't use more than > ~11-12GB. > > > ------------------------------ > *From:* Benedict Elliott Smith <belliottsm...@datastax.com> > *Sent:* Wednesday, June 4, 2014 2:07 PM > *To:* user@cassandra.apache.org > *Subject:* Re: memtable mem usage off by 10? > > I'm confused: there is no flush_largest_memtables_at property in C* 2.0? > > > On 4 June 2014 12:55, Idrén, Johan <johan.id...@dice.se> wrote: > >> Ok, so the overhead is a constant modifier, right. >> >> >> >> The 3x I arrived at with the following assumptions: >> >> >> >> heap is 10GB >> >> Default memory for memtable usage is 1/4 of heap in c* 2.0 >> max memory used for memtables is 2,5GB (10/4) >> >> flush_largest_memtables_at is 0.75 >> >> flush largest memtables when memtables use 7,5GB (3/4 of heap, 3x of the >> default) >> >> >> >> With an overhead of 10x, it makes sense that my memtable is flushed when >> the jmx data says it is at ~250MB, ie 2,5GB, ie 1/4 of the heap >> >> >> >> After I've set the memtable_total_size_in_mb to a value larger than >> 7,5GB, it should still not go over 7,5GB on account of >> flush_largest_memtables_at, 3/4 the heap >> >> >> >> So I would expect to see memtables flushed to disk after they're being >> reportedly at around 750MB. >> >> >> >> Having memtable_total_size_in_mb set to 20480, memtables are flushed at a >> reported value of ~2GB. >> >> >> >> With a constant overhead, this would mean that it used 20GB, which is 2x >> the size of the heap, instead of 3/4 of the heap as it should be if >> flush_largest_memtables_at was being respected. >> >> >> >> This shouldn't be possible. >> >> >> ------------------------------ >> *From:* Benedict Elliott Smith <belliottsm...@datastax.com> >> *Sent:* Wednesday, June 4, 2014 1:19 PM >> >> *To:* user@cassandra.apache.org >> *Subject:* Re: memtable mem usage off by 10? >> >> Unfortunately it looks like the heap utilisation of memtables was not >> exposed in earlier versions, because they only maintained an estimate. >> >> The overhead scales linearly with the amount of data in your memtables >> (assuming the size of each cell is approx. constant). >> >> flush_largest_memtables_at is an independent setting to >> memtable_total_space_in_mb, and generally has little effect. Ordinarily >> sstable flushes are triggered by hitting the memtable_total_space_in_mb >> limit. I'm afraid I don't follow where your 3x comes from? >> >> >> On 4 June 2014 12:04, Idrén, Johan <johan.id...@dice.se> wrote: >> >>> Aha, ok. Thanks. >>> >>> >>> >>> Trying to understand what my cluster is doing: >>> >>> >>> >>> cassandra.db.memtable_data_size only gets me the actual data but not >>> the memtable heap memory usage. Is there a way to check for heap memory >>> usage? >>> >>> >>> I would expect to hit the flush_largest_memtables_at value, and this >>> would be what causes the memtable flush to sstable then? By default 0.75? >>> >>> >>> Then I would expect the amount of memory to be used to be maximum ~3x of >>> what I was seeing when I hadn't set memtable_total_space_in_mb (1/4 by >>> default, max 3/4 before a flush), instead of close to 10x (250mb vs 2gb). >>> >>> >>> This is of course assuming that the overhead scales linearly with the >>> amount of data in my table, we're using one table with three cells in this >>> case. If it hardly increases at all, then I'll give up I guess :) >>> >>> At least until 2.1.0 comes out and I can compare. >>> >>> >>> BR >>> >>> Johan >>> >>> >>> ------------------------------ >>> *From:* Benedict Elliott Smith <belliottsm...@datastax.com> >>> *Sent:* Wednesday, June 4, 2014 12:33 PM >>> >>> *To:* user@cassandra.apache.org >>> *Subject:* Re: memtable mem usage off by 10? >>> >>> These measurements tell you the amount of user data stored in the >>> memtables, not the amount of heap used to store it, so the same applies. >>> >>> >>> On 4 June 2014 11:04, Idrén, Johan <johan.id...@dice.se> wrote: >>> >>>> I'm not measuring memtable size by looking at the sstables on disk, >>>> no. I'm looking through the JMX data. So I would believe (or hope) that I'm >>>> getting relevant data. >>>> >>>> >>>> >>>> If I have a heap of 10GB and set the memtable usage to 20GB, I would >>>> expect to hit other problems, but I'm not seeing memory usage over 10GB for >>>> the heap, and the machine (which has ~30gb of memory) is showing ~10GB >>>> free, with ~12GB used by cassandra, the rest in caches. >>>> >>>> >>>> >>>> Reading 8k rows/s, writing 2k rows/s on a 3 node cluster. So it's not >>>> idling. >>>> >>>> >>>> >>>> BR >>>> >>>> Johan >>>> >>>> >>>> ------------------------------ >>>> *From:* Benedict Elliott Smith <belliottsm...@datastax.com> >>>> *Sent:* Wednesday, June 4, 2014 11:56 AM >>>> *To:* user@cassandra.apache.org >>>> *Subject:* Re: memtable mem usage off by 10? >>>> >>>> If you are storing small values in your columns, the object overhead >>>> is very substantial. So what is 400Mb on disk may well be 4Gb in memtables, >>>> so if you are measuring the memtable size by the resulting sstable size, >>>> you are not getting an accurate picture. This overhead has been reduced by >>>> about 90% in the upcoming 2.1 release, through tickets 6271 >>>> <https://issues.apache.org/jira/browse/CASSANDRA-6271>, 6689 >>>> <https://issues.apache.org/jira/browse/CASSANDRA-6689> and 6694 >>>> <https://issues.apache.org/jira/browse/CASSANDRA-6694>. >>>> >>>> >>>> On 4 June 2014 10:49, Idrén, Johan <johan.id...@dice.se> wrote: >>>> >>>>> Hi, >>>>> >>>>> >>>>> >>>>> I'm seeing some strange behavior of the memtables, both in 1.2.13 and >>>>> 2.0.7, basically it looks like it's using 10x less memory than it should >>>>> based on the documentation and options. >>>>> >>>>> >>>>> >>>>> 10GB heap for both clusters. >>>>> >>>>> 1.2.x should use 1/3 of the heap for memtables, but it uses max ~300mb >>>>> before flushing >>>>> >>>>> >>>>> >>>>> 2.0.7, same but 1/4 and ~250mb >>>>> >>>>> >>>>> >>>>> In the 2.0.7 cluster I set the memtable_total_space_in_mb to 4096, >>>>> which then allowed cassandra to use up to ~400mb for memtables... >>>>> >>>>> >>>>> >>>>> I'm now running with 20480 for memtable_total_space_in_mb and >>>>> cassandra is using ~2GB for memtables. >>>>> >>>>> >>>>> >>>>> Soo, off by 10 somewhere? Has anyone else seen this? Can't find a JIRA >>>>> for any bug connected to this. >>>>> >>>>> java 1.7.0_55, JNA 4.1.0 (for the 2.0 cluster) >>>>> >>>>> >>>>> >>>>> BR >>>>> >>>>> Johan >>>>> >>>> >>>> >>> >>> >> >> > >
