On 10/24/2016 08:38 AM, Matus Kocka wrote:
Dear All, for some time we see ~20% regression in reread operation in iozone data. Kernel 4.6.0-1 was OK, but since that, there is ~20% regression from 1MB file size on SATA SSD disks and also similar one on SAS HDD, as you can see here: https://bugzilla.kernel.org/show_bug.cgi?id=155821 links to some plots: SAS HDD: https://bugzilla.kernel.org/attachment.cgi?id=242491 SATA SSD: https://bugzilla.kernel.org/attachment.cgi?id=242501 The plots are for the XFS file system, but it looks very similar for the Ext4. The machine which I used for the generating these plots is 2 NUMA, 32GB RAM Lenovo x3550 M5 I used numactl to pin iozone only to socket 0. We see this also on different machines. Can you help us to find the cause of this regression? Do you have any idea why it is only in reread? Data from the results, showing percentage difference between 4.6 and 4.7 kernels for various file size.
How fast is the storage for real? Since these graphs all have a cliff at 32GB, my guess is that we're really benchmarking how much of the file is kept in ram for all of the smaller sizes.
I'd suggest watching with blktrace (or even vmstat) to measure how much is read from disk during the smaller runs. If older kernels were doing better at keeping things in ram, that might explain the regression.
It could also be that reads are fighting with buffered writeback for access to the storage, but again vmstat and/or blktrace would help show that.
-chris
