Thanks Jeff. I'm trying to figure out why the tombstones scans are happening if possible eliminate it.

On Sat, Feb 23, 2019, 10:50 PM Jeff Jirsa <jjirsa@gmail.com> wrote:
G1GC with an 8g heap may be slower than CMS. Also you don’t typically set new gen size on G1.

Again though - what problem are you solving here? If you’re serving reads and sitting under 50% cpu, it’s not clear to me what you’re trying to fix. Tombstones scanned won’t matter for your table, so if that’s your only concern, I’d ignore it. 



-- 
Jeff Jirsa


On Feb 23, 2019, at 7:26 PM, Rahul Reddy <rahulreddy1234@gmail.com> wrote:

```jvm setting

-XX:+UseThreadPriorities
-XX:ThreadPriorityPolicy=42
-XX:+HeapDumpOnOutOfMemoryError
-Xss256k
-XX:StringTableSize=1000003
-XX:+AlwaysPreTouch
-XX:-UseBiasedLocking
-XX:+UseTLAB
-XX:+ResizeTLAB
-XX:+UseNUMA
-XX:+PerfDisableSharedMem
-Djava.net.preferIPv4Stack=true
-XX:+UseG1GC
-XX:G1RSetUpdatingPauseTimePercent=5
-XX:MaxGCPauseMillis=500
-XX:+PrintGCDetails
-XX:+PrintGCDateStamps
-XX:+PrintHeapAtGC
-XX:+PrintTenuringDistribution
-XX:+PrintGCApplicationStoppedTime
-XX:+PrintPromotionFailure
-XX:+UseGCLogFileRotation
-XX:NumberOfGCLogFiles=10
-XX:GCLogFileSize=10M

Total memory
free
             total       used       free     shared    buffers     cached
Mem:      16434004   16125340     308664         60     172872    5565184
-/+ buffers/cache:   10387284    6046720
Swap:            0          0          0

Heap settings in cassandra-env.sh
MAX_HEAP_SIZE="8192M"
HEAP_NEWSIZE="800M"
```

On Sat, Feb 23, 2019, 10:15 PM Rahul Reddy <rahulreddy1234@gmail.com> wrote:
Thanks Jeff,

Since low writes and high reads most of the time data in memtables only.  When I noticed intially issue no stables on disk everything in memtable only. 

On Sat, Feb 23, 2019, 10:01 PM Jeff Jirsa <jjirsa@gmail.com> wrote:
Also given your short ttl and low write rate, you may want to think about how you can keep more in memory - this may mean larger memtable and high flush thresholds (reading from the memtable), or perhaps the partition cache (if you are likely to read the same key multiple times). You’ll also probably win some with basic perf and GC tuning, but can’t really do that via email. Cassandra-8150 has some pointers. 

-- 
Jeff Jirsa


On Feb 23, 2019, at 6:52 PM, Jeff Jirsa <jjirsa@gmail.com> wrote:

You’ll only ever have one tombstone per read, so your load is based on normal read rate not tombstones. The metric isn’t wrong, but it’s not indicative of a problem here given your data model. 

You’re using STCS do you may be reading from more than one sstable if you update column2 for a given column1, otherwise you’re probably just seeing normal read load. Consider dropping your compression chunk size a bit (given the sizes in your cfstats I’d probably go to 4K instead of 64k), and maybe consider LCS or TWCS instead of STCS (Which is appropriate depends on a lot of factors, but STCS is probably causing a fair bit of unnecessary compactions and probably is very slow to expire data).

-- 
Jeff Jirsa


On Feb 23, 2019, at 6:31 PM, Rahul Reddy <rahulreddy1234@gmail.com> wrote:

Do you see anything wrong with this metric.

metric to scan tombstones
increase(cassandra_Table_TombstoneScannedHistogram{keyspace="mykeyspace",Table="tablename",function="Count"}[5m])

And sametime CPU Spike to 50% whenever I see high tombstone alert.

On Sat, Feb 23, 2019, 9:25 PM Jeff Jirsa <jjirsa@gmail.com> wrote:
Your schema is such that you’ll never read more than one tombstone per select (unless you’re also doing range reads / table scans that you didn’t mention) - I’m not quite sure what you’re alerting on, but you’re not going to have tombstone problems with that table / that select. 

-- 
Jeff Jirsa


On Feb 23, 2019, at 5:55 PM, Rahul Reddy <rahulreddy1234@gmail.com> wrote:

Changing gcgs didn't help

CREATE KEYSPACE ksname WITH replication = {'class': 'NetworkTopologyStrategy', 'dc1': '3', 'dc2': '3'}  AND durable_writes = true;


```CREATE TABLE keyspace."table" (
    "column1" text PRIMARY KEY,
    "column2" text
) WITH bloom_filter_fp_chance = 0.01
    AND caching = {'keys': 'ALL', 'rows_per_partition': 'NONE'}
    AND comment = ''
    AND compaction = {'class': 'org.apache.cassandra.db.compaction.SizeTieredCompactionStrategy', 'max_threshold': '32', 'min_threshold': '4'}
    AND compression = {'chunk_length_in_kb': '64', 'class': 'org.apache.cassandra.io.compress.LZ4Compressor'}
    AND crc_check_chance = 1.0
    AND dclocal_read_repair_chance = 0.1
    AND default_time_to_live = 18000
    AND gc_grace_seconds = 60
    AND max_index_interval = 2048
    AND memtable_flush_period_in_ms = 0
    AND min_index_interval = 128
    AND read_repair_chance = 0.0
    AND speculative_retry = '99PERCENTILE';

flushed table and took tsstabledump     
grep -i '"expired" : true' SSTables.txt|wc -l
16439
grep -i '"expired" : false'  SSTables.txt |wc -l
2657

ttl is 4 hours.

INSERT INTO keyspace."TABLE_NAME" ("column1", "column2") VALUES (?, ?) USING TTL(4hours) ?';
SELECT * FROM keyspace."TABLE_NAME" WHERE "column1" = ?';

metric to scan tombstones 
increase(cassandra_Table_TombstoneScannedHistogram{keyspace="mykeyspace",Table="tablename",function="Count"}[5m])

during peak hours. we only have couple of hundred inserts and 5-8k reads/s per node.
```

```tablestats
Read Count: 605231874
Read Latency: 0.021268529760215503 ms.
Write Count: 2763352
Write Latency: 0.027924007871599422 ms.
Pending Flushes: 0
Table: name
SSTable count: 1
Space used (live): 1413203
Space used (total): 1413203
Space used by snapshots (total): 0
Off heap memory used (total): 28813
SSTable Compression Ratio: 0.5015090954531143
Number of partitions (estimate): 19568
Memtable cell count: 573
Memtable data size: 22971
Memtable off heap memory used: 0
Memtable switch count: 6
Local read count: 529868919
Local read latency: 0.020 ms
Local write count: 2707371
Local write latency: 0.024 ms
Pending flushes: 0
Percent repaired: 0.0
Bloom filter false positives: 1
Bloom filter false ratio: 0.00000
Bloom filter space used: 23888
Bloom filter off heap memory used: 23880
Index summary off heap memory used: 4717
Compression metadata off heap memory used: 216
Compacted partition minimum bytes: 73
Compacted partition maximum bytes: 124
Compacted partition mean bytes: 99
Average live cells per slice (last five minutes): 1.0
Maximum live cells per slice (last five minutes): 1
Average tombstones per slice (last five minutes): 1.0
Maximum tombstones per slice (last five minutes): 1
Dropped Mutations: 0
histograms
Percentile  SSTables     Write Latency      Read Latency    Partition Size        Cell Count
                              (micros)          (micros)           (bytes)                  
50%             0.00             20.50             17.08                86                 1
75%             0.00             24.60             20.50               124                 1
95%             0.00             35.43             29.52               124                 1
98%             0.00             35.43             42.51               124                 1
99%             0.00             42.51             51.01               124                 1
Min             0.00              8.24              5.72                73                 0
Max             1.00             42.51            152.32               124                 1
```

3 node in dc1 and 3 node in dc2 cluster. With instanc type aws  ec2 m4.xlarge

On Sat, Feb 23, 2019, 7:47 PM Jeff Jirsa <jjirsa@gmail.com> wrote:
Would also be good to see your schema (anonymized if needed) and the select queries you’re running


-- 
Jeff Jirsa


On Feb 23, 2019, at 4:37 PM, Rahul Reddy <rahulreddy1234@gmail.com> wrote:

Thanks Jeff,

I'm having gcgs set to 10 mins and changed the table ttl also to 5  hours compared to insert ttl to 4 hours .  Tracing on doesn't show any tombstone scans for the reads.  And also log doesn't show tombstone scan alerts. Has the reads are happening 5-8k reads per node during the peak hours it shows 1M tombstone scans count per read. 

On Fri, Feb 22, 2019, 11:46 AM Jeff Jirsa <jjirsa@gmail.com> wrote:
If all of your data is TTL’d and you never explicitly delete a cell without using s TTL, you can probably drop your GCGS to 1 hour (or less).

Which compaction strategy are you using? You need a way to clear out those tombstones. There exist tombstone compaction sub properties that can help encourage compaction to grab sstables just because they’re full of tombstones which will probably help you.


-- 
Jeff Jirsa


On Feb 22, 2019, at 8:37 AM, Kenneth Brotman <kenbrotman@yahoo.com.invalid> wrote:

Can we see the histogram?  Why wouldn’t you at times have that many tombstones?  Makes sense.

 

Kenneth Brotman

 

From: Rahul Reddy [mailto:rahulreddy1234@gmail.com]
Sent: Thursday, February 21, 2019 7:06 AM
To: user@cassandra.apache.org
Subject: Tombstones in memtable

 

We have small table records are about 5k .

All the inserts comes as 4hr ttl and we have table level ttl 1 day and gc grace seconds has 3 hours.  We do 5k reads a second during peak load During the peak load seeing Alerts for tomstone scanned histogram reaching million.

Cassandra version 3.11.1. Please let me know how can this tombstone scan can be avoided in memtable