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From "Ibrahim El-sanosi (PGR)" <i.s.el-san...@newcastle.ac.uk>
Subject RE: Latency in asynchronous mode
Date Thu, 23 Oct 2014 22:31:56 GMT
WOW Kishore, You put the train on the tracks. 

Yes, you are absolutely right, your answer makes sense to me.

First of all, I concentrate on the latency no throughput. Therefore, it is better to use the
sync mode rather than using the async mode.  
Second, I did measurement of latency and throughput in both Async and Sync mode, the sample
result is as following: 

1- Sync mode with one client send CREATE requests last for 30 seconds:
Latency min/avg/max: 7/25/55
Throughput 224

1- Aync mode with one client send CREATE requests last for 30 seconds:
Latency min/avg/max: : 224/344/507
Throughput 3641

The above result supports your thought,  using single fsync (Sync mode) can low latency and
decease the throughput, whereas, in batch multiple requests (Async mode) the latency will
be higher and the throughput increase.

To conclude, I should use the Sync mode to test the performance because my work  focus in
latency.  Am I right?

Thank you

Ibrahim


-----Original Message-----
From: kishore g [mailto:g.kishore@gmail.com] 
Sent: Thursday, October 23, 2014 08:14 م
To: user@zookeeper.apache.org
Subject: Re: Latency in asynchronous mode

Async api is zookeeper is a way to achieve high through but by trading off latency. As others
have explained, before returning success to the user, zookeeper always ensures that the entry
is flushed into the transaction log. This operation is expensive can take around 5-20 ms on
spinning disk.
So if zookeeper followed a naive way of invoking fsync for every request it would be able
to handle approx 200 transaction per second at max( again depending on the fsync time). But
the server tries to do further optimization by trying to batch multiple requests in one fsync
also commonly known as group commit. This of course comes impacts the latency because each
request now has to incur some additional latency because the amount of data written to disk
is proportional to the batch size where as in the sync request each request would write the
data proportional to that request.

So coming back to your question, what you should be really measuring is the amortized latency
on a per write basis from the client. By using sync api, its unlikely that the group flush
is kicking in because the client waits for the ack of the previous request before sending
a new request. So you are seeing low latency but the total number of writes done during your
test would be low compared to the async api test. In case of async client is not really waiting
for ack for the write in the same thread. So all clients continue to send requests which means
there is a high chance of group flush to kick in on the server side and because of this the
perceived latency can be higher. For example, if you send 1000 requests back to back in async
you might see the latency as X ms for each request but if you measure the wall clock time
from the start of first request to the last ack got from the server it would be around X ms.

A good way to understand this is to measure both latency and throughput (total number of writes)
from your client. Also its not clear if the clients are trying to create new znodes or updating
the same one. If they are updating the same one there might be some conflicts that might create
additional latency. And are the clients doing read and write?. There are other design choices
in zookeeper such a single queue for both reads and writes that might impact latency as well.
It might help if you share the client code.

I think its important to understand your goal. In general there are two things one would like
to achieve low latency and high throughput. Achieving both is hard especially when it involves
disk io and fsync.

thanks,
Kishore G












On Thu, Oct 23, 2014 at 11:21 AM, Ibrahim El-sanosi (PGR) < i.s.el-sanosi@newcastle.ac.uk>
wrote:

> Hi Rakesh,
>
> First of all, the zookeeper ensemble consists of five Zookeeper servers.
> Also I have another 10 clients machines used to send write requests to 
> Zookeeper. The benchmark code creates 5 threads (equal to number of 
> Zookeeper server) , each thread associates with one Zookeeper server.  
> So, in this case, each zookeeper server will receive a set of write requests.
> The benchmark code runs for 30 seconds.
>
>  Async tests:
>
> * Number of clients
> In fact, I have different test, each test has different number of clients.
> For example, the bellow shows the latency corresponds to different 
> number of clients:
> Five clients: Latency min/avg/max: 235/366/515 Ten clients:  Latency 
> min/avg/max: 252/368/505
>
> * Number of threads
> As explained above, each client creates 5 threads and each thread 
> connects to one Zookeeper server. For instance, test using 5 clients’ 
> machines, each Zookeeper server receives five threads.
>
> * data size storing in each znode
> The data size store in znode is 100 bytes
>
> Also, it would be good to monitor :
>
> 1) JVM stats(one way is through JMX) like heap, gc activities. This is 
> to see if latency spike corresponds to gc activity or not.
>
> If you mean by JVM stats the four word stat command, then  the latency 
> result showed above is generated using this command. If you mean 
> something else then I have to read about and tell you late on.
>
> 2) Since you are doubting fsync, I think $ iostat would be helpful to 
> see disk statistics. For example, $ iostat -d -x 2 10 and collects the 
> disk latency.
>
> Yes, the batch size that I use in SyncrequestProcessor class is 1000 
> requests. I think this is preferable size. Also, I will try to use iostat.
>
> 3) CPU usage through top or sar unix commands. I didn't use sar , but 
> I could see it gives more details like percent of CPU running idle 
> with a process waiting for block I/O etc.
>
> Yes, I will use the top command to gathering the resource utilization.
> However, I don’t think top or sar will answer my question. Because I 
> am thinking there is different between Asynchroned and Synchronized 
> mode for measuring the latency.
>
> Thank you for your attention
>
> I look forward to hearing from you
>
>
> Ibrahim
>
> -----Original Message-----
> From: Rakesh Radhakrishnan [mailto:rakeshr.apache@gmail.com]
> Sent: Thursday, October 23, 2014 03:58 م
> To: user@zookeeper.apache.org
> Subject: Re: Latency in asynchronous mode
>
> Hi Ibrahim,
>
> In async tests, could you give the details like:
>
> * number of clients
> * number of threads
> * data size storing in each znode
>
> Also, it would be good to monitor :
>
> 1) JVM stats(one way is through JMX) like heap, gc activities. This is 
> to see if latency spike corresponds to gc activity or not.
>
> 2) Since you are doubting fsync, I think $ iostat would be helpful to 
> see disk statistics. For example, $ iostat -d -x 2 10 and collects the 
> disk latency.
>
> 3) CPU usage through top or sar unix commands. I didn't use sar , but 
> I could see it gives more details like percent of CPU running idle 
> with a process waiting for block I/O etc.
>
>
> -Rakesh
>
>
> On Thu, Oct 23, 2014 at 6:44 PM, Alexander Shraer <shralex@gmail.com>
> wrote:
>
> > Maybe due to queueing at the leader in asynchronous mode - if in 
> > your experiment you have one client in sync mode the leader has just 
> > one op in the queue at a time On Oct 23, 2014 1:57 PM, "Ibrahim"
> > <i.s.el-sanosi@newcastle.ac.uk> wrote:
> >
> > > Hi folks,
> > >
> > > I am testing ZooKeeper latency in Asynchronous mode. I am sending 
> > > update
> > > (write) requests to Zookeeper cluster that consists of 5 physical 
> > > Zookeeper.
> > >
> > > So, when I run the stat command I get high latency like:
> > > Latency min/avg/max: 7/339/392
> > > Latency min/avg/max: 1/371/627
> > > Latency min/avg/max: 1/371/627
> > > Latency min/avg/max: 1/364/674
> > > I guess such high latency correspond to fsync (batch requests). 
> > > But I
> > wish
> > > if someone could help me and explain this behaviour.
> > >
> > > However, testing Zookeeper using Synchronous mode, it gives me 
> > > reasonable result like:
> > > Latency min/avg/max: 6/24/55
> > > Latency min/avg/max: 7/22/61
> > > Latency min/avg/max: 7/30/65
> > >
> > > Note that the latency measures in milliseconds.
> > >
> > > I look forward to hearing from you.
> > >
> > > Ibrahim
> > >
> > >
> > >
> > >
> > >
> > >
> > >
> > > --
> > > View this message in context:
> > >
> > http://zookeeper-user.578899.n2.nabble.com/Latency-in-asynchronous-m
> > od
> > e-tp7580446.html
> > > Sent from the zookeeper-user mailing list archive at Nabble.com.
> > >
> >
>
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