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PO
 

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    <p>I'm guessing you're running into 'swapping'.</p> <p>As you'll know Linux swaps out some memory to disk. This is great for memory that's not accessed very much.</p> <p>When Java starts eating heaps and heaps, linux starts:</p> <ol> <li>Swapping memory block A out to disk to make space to read in block B</li> <li>Reading/writing block B</li> <li>Swapping block B to disk to make space for some other block.</li> </ol> <p>As disk is 1000s of times slower than RAM, as the memory usage increases your machine grinds more and more closer to a halt.</p> <p>With 256 MB Cloud Servers you get 512 MB of Swap space.</p> <hr> <p><strong>Checking:</strong></p> <p>You can check if this is the case with <code>free -m</code> .. <a href="http://www.linuxatemyram.com/" rel="nofollow">this page</a> shows how to read the output: </p> <p>Next I'd check with 'iostat 5' to see what the disk IO rate on the swap partition is. I would say a write rate of 300 or more means you're almost dead in the water. I'd say you'd want to keep the write rate of the swap partition down below 50 blocks a second and the read rate down below 500 blocks a second .. if possible both should be zero most of the time. Remember disk is 1000s of times slower than RAM.</p> <p>You can check if it's Java eating the ram by running <code>top</code> and hitting <code>shift+m</code> to order the processes by memory consumption.</p> <p>If you want .. you can disable the swap partition with <code>swapoff -a</code> .. then open up the web console, and hit the site a bit .. you'll soon see error messages in the console like 'OOM Killed process xxx' (OOM is for Out of Memory I think). If you see those that's linux trying to satisfy memory requests by killing processes. Once that happens, it's best to hard reboot.</p> <hr> <p><strong>Fixing:</strong></p> <p>If it's Java using the RAM .. <a href="http://publib.boulder.ibm.com/infocenter/wasinfo/v6r0/index.jsp?topic=/com.ibm.websphere.express.doc/info/exp/ae/rprf_javamemory.html" rel="nofollow">this link</a> might help.</p> <p>I think the easy fix would be just to upgrade the size of the Cloud Server.</p> <p>You may find a different Java RTE may be better.</p> <p>If you run it in <a href="http://ubuntuforums.org/showthread.php?t=24575" rel="nofollow">a 32 bit chroot</a> it may use less RAM.</p>
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    1. COHey, thank you! This is a great response. Could you clarify the 'write rate of 300 or more', and how do I know which is the swap partition? By running iostat, the output shows me that I have 2 devices, xvdc and xvda. xvdc is doing most of the work, with usually tps over 1000, kB_read/s over 3000, kB_write/s over 1000. But overall, I think I know where my problem is. I will try upgrading my server tonight and let you know if it worked.
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    2. COIt was PermGen that's also taking up the memory, thanks for your help!
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    3. COI'm pretty sure xvdc will be your swap partition. You can see by running: sudo grep xvd /etc/fstab; 3000 writes a second is way too high .. that should really be killing your performance. Definitely too much memory usage.
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