On Thu, 24 Jul 2008, James Bottomley wrote: > On Thu, 2008-07-24 at 12:34 -0400, Mikulas Patocka wrote: > > On Thu, 24 Jul 2008, James Bottomley wrote: > > > > > On Thu, 2008-07-24 at 11:07 -0400, Mikulas Patocka wrote: > > > > So try to #define BIO_VMERGE_BOUNDARY 0 for Pa-Risc and tell us what > > > > performance degradation do you see (and what driver do you use and what is > > > > the I/O pattern). > > > > > > > > If you show something specific, we can consider that --- but you haven't > > > > yet told us anything, except generic talk. > > > > > > You keep ignoring inconvenient facts. For about the third time: > > > > > > I run a test bed for sg_tables (large chaining of requests). This runs > > > on parisc using virtual merging (has to because the final physical table > > > size can't go over the sg list of the SCSI card). If I turn off virtual > > > merging I can no longer test sg_tables in vanilla kernels. > > > > > > James > > > > What sg_tables test do you mean? What does the test do? Why couldn't you > > run the test if BIO_VMERGE_BOUNDARY is 0? Normal I/O obviously can work > > with BIO_VMERGE_BOUNDARY 0, the kernel will just send more smaller > > Look, if you don't really understand what I'm doing, it's not really my > job to educate you. The sg_table discussions are on marc.info, mainly > on the SCSI lists; just look for 'sg chaining' in the header (need to > use google site ... marc's search is bad). > > You can complain if the code is impacting you ... but I believe I've > optimised it so it isn't. Your basic problem amounts to you not liking > me doing something that has no impact on you ... I'm afraid that's what > freedom leads to (shocking, I know). > > James Chaining of sg_tables is used for drivers with big sg tables --- and vmerge counting is used for drivers with small sg tables. So what do they have in common? Summary, what I mean: * in blk-merge.c, you have 85 lines, that is 16% of the size of the file, devoted to counting of hw_segments * it is only used on two architectures, one already outdated (alpha), the other being discontinued (pa-risc). On all the other architectures, hw_segments == phys_segments * it is prone to bugs and hard to maintain, because the same value must be calculated in blk-merge.c and in architectural iommu functions --- if the value differs, you create too long request, corrupt kernel memory and crash (happened on sparc64). Anyone changing blk-merge in the future will risk breaking something on the architectures that use BIO_VMERGE_BOUNDARY --- and because these architectures are so rare, the bug will go unnoticed for long time --- like in the case of sparc64. * you are just talking how this code is important for performance without showing any single proof that it really is (temporarily disable hw_segments accounting by defining BIO_VMERGE_BOUNDARY 0 and get the numbers). Mikulas -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
