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From "Liu, Ming (HPIT-GADSC)" <ming.l...@hp.com>
Subject RE: how to explain read/write performance change after modifying the hfile.block.cache.size?
Date Fri, 21 Nov 2014 05:55:25 GMT
Thank you Lars,

There must be something wrong with my testing yesterday. I cannot reproduce the issue anymore.
Now, changing the cache.size from 0 to 0.4 will not slow down the write perf dramatically,
but still will slow down write (Yesterday I saw a 7x slowdown, today it is about 1.3x slowdown
which is acceptable for me). As Ted pointed out, it is possible that memstore cannot get enough
memory when more RAM give to block cache so it flush more frequently, but I really need more
reading to understand when memstore will flush.
And one thing I noticed is when I restart hbase for the very first test, the performance is
best, then the second time, it is slower, both read and write, and slower and slower in the
following tests and get to a stable point after about 3 or 4 times, in each run I will read
5,000,000 rows and update 5,000,000 rows. There are too many factors affect the read/write
OPS in hbase...

My purpose is to find a proper way to evaluate performance, since we are going to change something
in hbase and it is good to have a base benchmark so we can compare the performance after change.
So I must make sure the perf test itself make sense and should be trusted. 

I saw an entry in the log may help to see the cache settings in my system:
hfile.LruBlockCache: Total=373.54 MB, free=13.13 MB, max=386.68 MB, blocks=5655, accesses=17939947,
hits=14065015, hitRatio=78.40%, , cachingAccesses=17934857, cachingHits=14064420, cachingHitsRatio=78.42%,
evictions=15646, evicted=3861343, evictedPerRun=246.7942657470703

My testing environment is a workstation with 12 core CPU, 96G memory and 1.7 T disk. But it
is a shared workstation, many users share it and I started hbase in standalone mode with hbase-site.xml
as below :

     <value>90000000</value>    :-) I just want to make sure never timeout here,
since I get timeout so many times...

[liuliumi@ YCSB]$ free -m
             total       used       free     shared    buffers     cached
Mem:         96731      46828      49903          0        984      32525
-/+ buffers/cache:      13317      83413
Swap:        48432         12      48420

[liuliumi@ YCSB]$ lscpu
Architecture:          x86_64
CPU op-mode(s):        32-bit, 64-bit
Byte Order:            Little Endian
CPU(s):                12
On-line CPU(s) list:   0-11
Thread(s) per core:    1
Core(s) per socket:    6
Socket(s):             2
NUMA node(s):          2
Vendor ID:             GenuineIntel
CPU family:            6
Model:                 44
Stepping:              2
CPU MHz:               3065.894
BogoMIPS:              6130.96
Virtualization:        VT-x
L1d cache:             32K
L1i cache:             32K
L2 cache:              256K
L3 cache:              12288K
NUMA node0 CPU(s):     0,2,4,6,8,10
NUMA node1 CPU(s):     1,3,5,7,9,11


-----Original Message-----
From: lars hofhansl [mailto:larsh@apache.org] 
Sent: Friday, November 21, 2014 4:31 AM
To: user@hbase.apache.org
Subject: Re: how to explain read/write performance change after modifying the hfile.block.cache.size?

That would explain it if memstores are flushed due to global memory pressure.

But cache and memstore size are (unfortunately) configured independently. The memstore heap
portion would be 40% (by default) in either case.So this is a bit curious still.
Ming, can you tell us more details?- RAM on the boxes- heap setup for the region servers-
any other relevant settings on hbase-site.xml- configs on the table/column family you're writing
to (like bloom filters, etc).

That would help us diagnose this.

-- Lars

      From: Ted Yu <yuzhihong@gmail.com>
 To: "user@hbase.apache.org" <user@hbase.apache.org>
 Sent: Thursday, November 20, 2014 9:32 AM
 Subject: Re: how to explain read/write performance change after modifying the hfile.block.cache.size?
When block cache size increases from 0 to 0.4, the amount of heap given to memstore decreases.
This would slow down the writes.
Please see:

For your second question, see this thread:


On Thu, Nov 20, 2014 at 8:05 AM, Liu, Ming (HPIT-GADSC) <ming.liu2@hp.com>

> Hello, all,
> I am playing with YCSB to test HBase performance. I am using HBase 0.98.5.
> I tried to adjust the hfile.block.cache.size to see the difference, 
> when I set hfile.block.cache.size to 0, read performance is very bad, 
> but write performance is very very very good....; when I set 
> hfile.block.cache.size to 0.4, read is better, but write performance 
> drop dramatically. I disable the client side writebuffer already.
> This is hard to understand for me:
> The HBase guide just said hfile.block.cache.size setting is about how 
> much memory used as block cache used by StoreFile. I have no idea of 
> how HBase works internally. Typically, it is easy to understand that 
> increase the size of cache should help the read, but why it will harm 
> the write operation? The write performance down from 30,000 to 4,000 
> for your reference, just by changing the hfile.block.cache.size from 0 to 0.4.
> Could anyone give me a brief explanation about this observation or 
> give me some advices about what to study to understand what is block cache used for?
> Another question: HBase write will first write to WAL then to memstore.
> Will the write to WAL go to disk directly before hbase write memstore, 
> a sync operation or it is possible that write to WAL is still buffered 
> somewhere when hbase put the data into the memstore?
> Reading src code may cost me months, so a kindly reply will help me a 
> lot... ...
> Thanks very much!
> Best Regards,
> Ming

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