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    SubjectRe: Strange IDE performance change in 2.6.1-rc1 (again)
    On Saturday 10 January 2004 17:19, Ed Sweetman wrote:
    > debian unstable's dd may also be seeing that it's writing to /dev/null
    > and just not doing anything. I know extents are fast and make certain
    > manipulations on them extremely faster than plain ext3 but 256MB/sec is
    > really really too fast. So in either case it looks like this test is not
    > usable to me.

    yes... 256MB/s is a bit too high!
    Can you try with "fadvice" installed?
    Anyway I think your theroy is right... and so intalling "fadvice" you will
    NOT see any big difference.

    > I dont know why you dont also try 8192 for readahead, measuring

    beacuse readahead setted to 8192 gives me BAD performance!

    > performance by the duration or intensity of the hdd is led is not very
    > sound. i actually copy large files to and from parts of the same ext3
    > partition at over 20MB/sec sustained hdparm shows it's highest numbers
    > under it. For me it doesn't get any faster than that. So what's this
    > all say, maybe all these performance numbers are just as much based on
    > your readahead value as they are on the position of the moon and the
    > rest of the system and it's hardware. btw, what is the value of your HZ
    > environment variable, debian still sets it to 100, i set it to 1024, not
    > really sure if it made any difference.
    > i'm using the via ide driver, so are you, i'm not seeing the type of
    > regression that you are, my dd doesn't do what your dd does. our hdds
    > are different. The regression in the kernels could just as easily be
    > due to a regression in the schedular and nothing to do with the ide
    > drivers. Have you tried just using 2.6.0 (whatever version you see
    > changes with your readahead values) then the same kernel with the new
    > ide code from the kernel you dont see any changes so you're running
    > everything else the same but only ide has been "upgraded" and see if you
    > see the same regression. I dont think you will. the readahead effects

    Yes, the correct way to work is as you say....
    BUT read the whole story:

    1) using "hdparm -t /dev/hda" I found IDE performace regression (in
    sequential reads) upgrading from 2.6.0 to 2.6.1-rc1

    2) someone tell me to try to revert this patch:
    "readahead: multiple performance fixes"

    Reverting it in 2.6.1-rc1 kernel gives me the same ide performaces that
    2.6.0 has.

    3) Since 2.6.0 and 2.6.1-rc1(with "readahead: multiple performance fixes"
    reverted) kernels give me the same results for any IDE performance test I
    do --> I treat them as they are the same thing ;-)

    The part of the patch that gives me all these problem is already found and
    is quite small:

    diff -Nru a/mm/filemap.c b/mm/filemap.c

    --- a/mm/filemap.c Sat Jan 3 02:29:08 2004
    +++ b/mm/filemap.c Sat Jan 3 02:29:08 2004
    @@ -587,13 +587,22 @@
    read_actor_t actor)
    struct inode *inode = mapping->host;
    - unsigned long index, offset;
    + unsigned long index, offset, last;
    struct page *cached_page;
    int error;

    cached_page = NULL;
    index = *ppos >> PAGE_CACHE_SHIFT;
    offset = *ppos & ~PAGE_CACHE_MASK;
    + last = (*ppos + desc->count) >> PAGE_CACHE_SHIFT;
    + /*
    + * Let the readahead logic know upfront about all
    + * the pages we'll need to satisfy this request
    + */
    + for (; index < last; index++)
    + page_cache_readahead(mapping, ra, filp, index);
    + index = *ppos >> PAGE_CACHE_SHIFT;

    for (;;) {
    struct page *page;
    @@ -612,7 +621,6 @@

    - page_cache_readahead(mapping, ra, filp, index);

    nr = nr - offset;

    Paolo Ornati
    Linux v2.4.24

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