Re: Reduce memcpy in glfs read and write

[Mohammed Rafi K C <rkavunga@xxxxxxxxxx>]

Hi Sachin,

Good to see you again in Gluster-devel.

I had implemented those API which I mentioned for a POC. In fact I couldn't push into master.

Regarding your questions, my comments are inline.

On 06/22/2016 05:50 AM, Sachin Pandit wrote:

Hey Pranith, I am good, I hope you are doing good too.

Please find the comments inline.

 

From: Pranith Kumar Karampuri [mailto:pkarampu@xxxxxxxxxx]
Sent: Tuesday, June 21, 2016 5:58 AM
To: Sachin Pandit <spandit@xxxxxxxxxxxxx>
Cc: gluster-devel@xxxxxxxxxxx
Subject: Re: Reduce memcpy in glfs read and write

 

Hey!!

       Hope you are doing good. I took a look at the bt. So when flush comes write-behind has to flush all the writes down. I see the following frame hung in iob_unref:
Thread 7 (Thread 0x7fa601a30700 (LWP 16218)):
#0  0x00007fa60cc55225 in pthread_spin_lock () from /lib64/libpthread.so.0  <<---- Does it always hang there?

---------------------------------

>>It does always hang here.

---------------------------------
#1  0x00007fa60e1f373e in iobref_unref (iobref=0x19dc7e0) at iobuf.c:907
#2  0x00007fa60e246fb2 in args_wipe (args=0x19e70ec) at default-args.c:1593
#3  0x00007fa60e1ea534 in call_stub_wipe_args (stub=0x19e709c) at call-stub.c:2466
#4  0x00007fa60e1ea5de in call_stub_destroy (stub=0x19e709c) at call-stub.c:2482

Is this on top of master branch? It seems like we missed an unlock of the spin-lock or the iobref has junk value which gives the feeling that it is in locked state (May be double free?). Do you have any extra patches you have in your repo which make changes in iobuf?

----------------------------------

>>I have implemented a method to reduce memcpy in libgfapi (My patch is on top of master branch), by making use of buffer from iobuf pool and passing the buffer to application. However, I have not made any changes in iobuf core feature. I don’t  think double free is happening anywhere in the code (I did check this using logs)

 

     Method that I have implemented:

1)      Application asks for a buffer of specific size, and the buffer is allocated from the iobuf pool.

2)      Buffer is passed on to application, and the application writes the data into that buffer.

3)      Buffer with data in it is passed from application to libgfapi and the underlying translators (no memcpy in glfs_write)

 

I have couple of questions, and observations:

 

Observations:

------------------

1)      For every write if I get a fresh buffer then I don’t see any problem. All the writes are going through.

2)      If I try to make use of buffer for consecutive writes, then I am seeing the hang in flush.

 

Question1: Is it fine if I reuse the buffer for consecutive writes??

The answer is no, If IO-cache and write-behind are enabled. In case of IO-cache, It just take a ref on the buffer to store into the cache, which means the io-cache is still using the buffer,

In case of write-behind, if it decided to aggregate multiple write request , in this case also it will take a ref on the iobuf and will lie to the appluca

Question2: Is it always ensured that the data is written to the file when I get a response from syncop_writev.

As I explained the write-behind may prevent this.

 

Thanks,

Sachin Pandit.

 

----------------------------------

 

On Tue, Jun 21, 2016 at 4:07 AM, Sachin Pandit <spandit@xxxxxxxxxxxxx> wrote:

Hi all,

 

I bid adieu to you all with the hope of crossing path again, and the time has come rather quickly. It feels great to work on GlusterFS again.

               

Currently we are trying to write data backed up by Commvault Simpana to glusterfs volume (Disperse volume). To improve the performance, I have implemented the proposal put forward my Rafi  K C [1]. I have some questions regarding libgfapi and iobuf pool.

 

To reduce an extra level of copy in glfs read and write, I have implemented few APIs to request a buffer (similar to the one represented in  [1]) from iobuf pool which can be used by the application to write data to. With this implementation, when I try to reuse the buffer for consecutive writes, I could see a hang in syncop_flush of glfs_close (BT of the hang can be found in [2]). I wanted to know if reusing the buffer is recommended. If not, do we need to request buffer for each writes?

 

Setup : Distributed-Disperse ( 4 * (2+1)). Bricks scattered over 3 nodes.

 

[1] http://www.gluster.org/pipermail/gluster-devel/2015-February/043966.html

[2] Attached file -  bt.txt

 

Thanks & Regards,

Sachin Pandit.

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--

Pranith

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