Hello, Is anybody aware of any changes in the 2.4.21 kernel which affects disk I/O badly? We are noticing that the cpu taken up by iowait's is causing significant degradation in disk performance. We have an application in which there are multiple processes writing to disk simultaneously, each process writing anywhere from 16 - 64 K Bytes at a time and sleeping for about 50 ms depending on how long the write() call takes to finish. It is important that the write() finishes within 50 ms for this application. The file's are opened with the open() call with the standard O_RDWR and O_CREAT flags. On the 2.4.18 kernel, 25 of these processes take up about 15% of CPU on a dual Xeon Pentium 4 2.4 GHz processor with 1 MB of memory for a total throughput of about 7 MBps which is very good. The write() calls typically take less than 5 ms to complete and we sleep for the remaining 45 ms or so. But on the same system when we installed the 2.4.21 kernel, these 25 processes take anywhere between 40 - 70 % of the CPU depending on how much CPU the iowait is taking up. The iowait part of it varies all the way from 10 - 60 %. (The iowait CPU is within 1% on the 2.4.18 kernel.). What is even worse - sometimes the write() calls take anywhere between 1 - 2 seconds( yes seconds!) to return, which degrades the performance of our application server pretty badly. It happens on all kinds of storage devices so it's definitely not a hardware problem.(We tested on the standard IDE disk all the way upto the EMC clariion storage system). Is this iowait something which was introduced in the 2.4.21 kernel core code or in the file system driver code? It happens on reiserfs and jfs as also on ext3 too, though not as badly. Is there a way to turn it off or to make it take up lesser CPU? Thank you for your support. - Suman Puthana PS - Something else of interest here is that when the iowait part of the CPU increases, iostat shows a significantly higher throughput than what we are actually trying to write to disk. (When sampled over 30 seconds, when we are trying to write 7 MBps, iostat actually shows 11-12 MBps when iowait goes upto 30-40 %). It's almost like there's some internal copies going on. -- Kernelnewbies: Help each other learn about the Linux kernel. Archive: http://mail.nl.linux.org/kernelnewbies/ FAQ: http://kernelnewbies.org/faq/
