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
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, 6689 and 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
