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    SubjectRe: [PATCH] block: fix q->max_segment_size checking in blk_recalc_rq_segments about VMERGE
    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    * it is only used on two architectures, one already outdated (alpha), the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


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