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    SubjectRe: O_DIRECT question
    On 1/11/07, Linus Torvalds <torvalds@osdl.org> wrote:
    >
    > The "good" news is that CPU really is outperforming disk more and more, so
    > the extra cost of managing the page cache keeps on getting smaller and
    > smaller, and (fingers crossed) some day we can hopefully just drop
    > O_DIRECT and nobody will care.
    >
    > Linus
    >
    Yes for desktop, server, but maybe not for embedded system, specially
    for no-mmu linux. In many embedded system cases, the whole system is
    running in the ram, including file system. So it's not necessary using
    page cache anymore. Page cache can't improve performance on these
    cases, but only fragment memory.
    Maybe O_DIRECT is not a right way to fix this issue. But I think file
    system need an option for un-buffered access, that means don't use
    page cache at all.

    -Aubrey

    P.S. The following is the test case and crash info. I think it will
    help what exactly I encountered.
    ------------------------------------
    #include <stdio.h>
    #include <stdlib.h>
    #define N 8

    int main (void){
    void *p[N];
    int i;

    printf("Alloc %d MB !\n", N);

    for (i = 0; i < N; i++) {
    p[i] = malloc(1024 * 1024);
    if (p[i] == NULL)
    printf("alloc failed\n");
    }

    printf("alloc successful \n");
    for (i = 0; i < N; i++)
    free(p[i]);
    }
    --------------------------------------------------------------

    When there is not enough free memory to allocate:
    ==============================
    root:/mnt> cat /proc/meminfo
    MemTotal: 54196 kB
    MemFree: 5520 kB <== only 5M free
    Buffers: 76 kB
    Cached: 44696 kB <== cache eat 40MB
    SwapCached: 0 kB
    Active: 21092 kB
    Inactive: 23680 kB
    HighTotal: 0 kB
    HighFree: 0 kB
    LowTotal: 54196 kB
    LowFree: 5520 kB
    SwapTotal: 0 kB
    SwapFree: 0 kB
    Dirty: 0 kB
    Writeback: 0 kB
    AnonPages: 0 kB
    Mapped: 0 kB
    Slab: 3720 kB
    PageTables: 0 kB
    NFS_Unstable: 0 kB
    Bounce: 0 kB
    CommitLimit: 27096 kB
    Committed_AS: 0 kB
    VmallocTotal: 0 kB
    VmallocUsed: 0 kB
    VmallocChunk: 0 kB
    ==========================================


    I got failure after run the test program.
    ---------------------------------------
    root:/mnt> ./t
    Alloc 8 MB !
    t: page allocation failure. order:9, mode:0x40d0
    Hardware Trace:
    0 Target : <0x00004de0> { _dump_stack + 0x0 }
    Source : <0x0003054a> { ___alloc_pages + 0x17e }
    1 Target : <0x0003054a> { ___alloc_pages + 0x17e }
    Source : <0x0000dbc2> { _printk + 0x16 }
    2 Target : <0x0000dbbe> { _printk + 0x12 }
    Source : <0x0000da4e> { _vprintk + 0x1a2 }
    3 Target : <0x0000da42> { _vprintk + 0x196 }
    Source : <0xffa001ea> { __common_int_entry + 0xd8 }
    4 Target : <0xffa00188> { __common_int_entry + 0x76 }
    Source : <0x000089bc> { _return_from_int + 0x58 }
    5 Target : <0x000089bc> { _return_from_int + 0x58 }
    Source : <0x00008992> { _return_from_int + 0x2e }
    6 Target : <0x00008964> { _return_from_int + 0x0 }
    Source : <0xffa00184> { __common_int_entry + 0x72 }
    7 Target : <0xffa00182> { __common_int_entry + 0x70 }
    Source : <0x00012682> { __local_bh_enable + 0x56 }
    8 Target : <0x0001266c> { __local_bh_enable + 0x40 }
    Source : <0x0001265c> { __local_bh_enable + 0x30 }
    9 Target : <0x00012654> { __local_bh_enable + 0x28 }
    Source : <0x00012644> { __local_bh_enable + 0x18 }
    10 Target : <0x0001262c> { __local_bh_enable + 0x0 }
    Source : <0x000128e0> { ___do_softirq + 0x94 }
    11 Target : <0x000128d8> { ___do_softirq + 0x8c }
    Source : <0x000128b8> { ___do_softirq + 0x6c }
    12 Target : <0x000128aa> { ___do_softirq + 0x5e }
    Source : <0x0001666a> { _run_timer_softirq + 0x82 }
    13 Target : <0x000165fc> { _run_timer_softirq + 0x14 }
    Source : <0x00023eb8> { _hrtimer_run_queues + 0xe8 }
    14 Target : <0x00023ea6> { _hrtimer_run_queues + 0xd6 }
    Source : <0x00023e70> { _hrtimer_run_queues + 0xa0 }
    15 Target : <0x00023e68> { _hrtimer_run_queues + 0x98 }
    Source : <0x00023eae> { _hrtimer_run_queues + 0xde }
    Stack from 015a7dcc:
    00000001 0003054e 00000000 00000001 000040d0 0013c70c 00000009 000040d0
    00000000 00000080 00000000 000240d0 00000000 015a6000 015a6000 015a6000
    00000010 00000000 00000001 00036e12 00000000 0023f8e0 00000073 00191e40
    00000020 0023e9a0 000040d0 015afea9 015afe94 00101fff 000040d0 0023e9a0
    00000010 00101fff 000370de 00000000 0363d3e0 00000073 0000ffff 04000021
    00000000 00101000 00187af0 00035b44 00000000 00035e40 00000000 00000000
    Call Trace:
    Call Trace:
    [<0000fffe>] _do_exit+0x12e/0x7cc
    [<00004118>] _sys_mmap+0x54/0x98
    [<00101000>] _fib_create_info+0x670/0x780
    [<00008828>] _system_call+0x68/0xba
    [<000040c4>] _sys_mmap+0x0/0x98
    [<0000fffe>] _do_exit+0x12e/0x7cc
    [<00008000>] _cplb_mgr+0x8/0x2e8
    [<00101000>] _fib_create_info+0x670/0x780
    [<00101000>] _fib_create_info+0x670/0x780

    Mem-info:
    DMA per-cpu:
    cpu 0 hot: high 18, batch 3 used:5
    cpu 0 cold: high 6, batch 1 used:5
    DMA32 per-cpu: empty
    Normal per-cpu: empty
    HighMem per-cpu: empty
    Free pages: 21028kB (0kB HighMem)
    Active:2549 inactive:3856 dirty:0 writeback:0 unstable:0 free:5257
    slab:1833 mapped:0 pagetables:0
    DMA free:21028kB min:948kB low:1184kB high:1420kB active:10196kB
    inactive:15424kB present:56320kB pages_scanned:0 all_unreclaimable? no
    lowmem_reserve[]: 0 0 0 0
    DMA32 free:0kB min:0kB low:0kB high:0kB active:0kB inactive:0kB
    present:0kB pages_scanned:0 all_unreclaimable? no
    lowmem_reserve[]: 0 0 0 0
    Normal free:0kB min:0kB low:0kB high:0kB active:0kB inactive:0kB
    present:0kB pages_scanned:0 all_unreclaimable? no
    lowmem_reserve[]: 0 0 0 0
    HighMem free:0kB min:128kB low:128kB high:128kB active:0kB
    inactive:0kB present:0kB pages_scanned:0 all_unreclaimable? no
    lowmem_reserve[]: 0 0 0 0
    DMA: 43*4kB 35*8kB 28*16kB 17*32kB 18*64kB 20*128kB 16*256kB 11*512kB
    6*1024kB 0*2048kB 0*4096kB 0*8192kB 0*16384kB 0*32768kB = 21028kB
    DMA32: empty
    Normal: empty
    HighMem: empty
    14080 pages of RAM
    5285 free pages
    531 reserved pages
    11 pages shared
    0 pages swap cached
    Allocation of length 1052672 from process 57 failed
    DMA per-cpu:
    cpu 0 hot: high 18, batch 3 used:5
    cpu 0 cold: high 6, batch 1 used:5
    DMA32 per-cpu: empty
    Normal per-cpu: empty
    HighMem per-cpu: empty
    Free pages: 21028kB (0kB HighMem)
    Active:2549 inactive:3856 dirty:0 writeback:0 unstable:0 free:5257
    slab:1833 mapped:0 pagetables:0
    DMA free:21028kB min:948kB low:1184kB high:1420kB active:10196kB
    inactive:15424kB present:56320kB pages_scanned:0 all_unreclaimable? no
    lowmem_reserve[]: 0 0 0 0
    DMA32 free:0kB min:0kB low:0kB high:0kB active:0kB inactive:0kB
    present:0kB pages_scanned:0 all_unreclaimable? no
    lowmem_reserve[]: 0 0 0 0
    Normal free:0kB min:0kB low:0kB high:0kB active:0kB inactive:0kB
    present:0kB pages_scanned:0 all_unreclaimable? no
    lowmem_reserve[]: 0 0 0 0
    HighMem free:0kB min:128kB low:128kB high:128kB active:0kB
    inactive:0kB present:0kB pages_scanned:0 all_unreclaimable? no
    lowmem_reserve[]: 0 0 0 0
    DMA: 43*4kB 35*8kB 28*16kB 17*32kB 18*64kB 20*128kB 16*256kB 11*512kB
    6*1024kB 0*2048kB 0*4096kB 0*8192kB 0*16384kB 0*32768kB = 21028kB
    DMA32: empty
    Normal: empty
    HighMem: empty
    -----------------------------

    If there is no page cache, I have another 40Mb to run the test
    program. I'm pretty sure the program can work properly at the first
    time.
    -
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