James Bottomley, on 04/02/2009 12:23 AM wrote: > On Wed, 2009-04-01 at 08:20 -0400, Ross Walker wrote: >> On Apr 1, 2009, at 2:29 AM, Bart Van Assche <bart.vanassche at gmail.com> >> wrote: >> >>> On Tue, Mar 31, 2009 at 8:43 PM, Ross S. W. Walker >>> <RWalker at medallion.com> wrote: >>>> IET just needs to fix how it does it workload with CFQ which >>>> somehow SCST has overcome. Of course SCST tweaks the Linux kernel to >>>> gain some extra speed. >>> I'm not familiar with the implementation details of CFQ, but I know >>> that one of the changes between SCST 1.0.0 and SCST 1.0.1 is that the >>> default number of kernel threads of the scst_vdisk kernel module has >>> been increased to 5. Could this explain the performance difference >>> between SCST and IET for FILEIO and BLOCKIO ? >> Thank for the update. IET has used 8 threads per target for ages now, >> I don't think it is that. >> >> It may be how the I/O threads are forked in SCST that causes them to >> be in the same I/O context with each other. >> >> I'm pretty sure implementing a version of the patch that was used for >> the dump command (found on the LKML) will fix this. >> >> But thanks goes to Vlad for pointing this dificiency out so we can fix >> it to help make IET even better. > > SCST explicitly fiddles with the io context to get this to happen. It > has a hack to block to export alloc_io_context: > > http://marc.info/?t=122893564800003 Correct, although I wouldn't call it "fiddle", rather "grouping" ;) But that's not the only reason for good performance. Particularly, it can't explain Bart's tmpfs results from the previous message, where the majority of I/O done to/from RAM without any I/O scheduler involved. (Or does I/O scheduler also involved with tmpfs?) Bart has 4GB RAM, if I remember correctly, i.e. the test data set was 25% of RAM. Thanks, Vlad -- To unsubscribe from this list: send the line "unsubscribe stgt" in the body of a message to majordomo at vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html |