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    SubjectRe: Memory Problem in 2.4.10-pre2 / __alloc_pages failed
    On Thursdayen den 30 August 2001 01:36, Daniel Phillips wrote:
    > On August 29, 2001 02:07 pm, Stephan von Krawczynski wrote:
    > > I managed it again. As with previous 2.4-releases I managed to let
    > > __alloc_pages fail quite easily with pretty standard test bed:
    > >
    > > Hardware: 2 x P-III 1GHz, 1 GB RAM, 29160 U160 SCSI, 36GB HD
    > > Kernel: 2.4.10-pre2 with trace output in mm/page_alloc.c (thanks Roger)
    > >
    > > Test:
    > > exported reiserfs filesystem, simply copying files on it from a 2.2.19
    > > nfs client (files are big 10-20 MB each). at the same time I read a CD to
    > > HD via xcdroast and run setiathome on nice-level 19.
    > >
    > > Everything runs acceptable, but CPU-Load is high (6-8). Simply "cat
    > > /proc/meminfo" takes half a minute sometimes during test. Mouse cannot be
    > > moved smoothly during the whole test. When xcdroast is finished with
    > > reading CD (at about 1 MB/sec speed, not very fast indeed) the following
    > > shows up:
    > >
    > > Aug 29 13:43:34 admin kernel: __alloc_pages: 1-order allocation failed
    > > (gfp=0x20/0). Aug 29 13:43:34 admin kernel: pid=1207;
    > > __alloc_pages(gfp=0x20, order=1, ...) Aug 29 13:43:34 admin kernel: Call
    > > Trace: [_alloc_pages+22/24] [__get_free_pages+10/24] [<fdcec845>]
    > > [<fdcec913>] [<fdceb7d7>] Aug 29 13:43:34 admin kernel: [<fdcec0f5>]
    > > [<fdcea589>] [ip_local_deliver_finish+0/368] [nf_hook_slow+272/404]
    > > [ip_rcv_finish+0/480] [ip_local_deliver+436/444] Aug 29 13:43:34 admin
    > > kernel: [ip_local_deliver_finish+0/368] [ip_rcv_finish+0/480]
    > > [ip_rcv_finish+413/480] [ip_rcv_finish+0/480] [nf_hook_slow+272/404]
    > > [ip_rcv+870/944] Aug 29 13:43:34 admin kernel: [ip_rcv_finish+0/480]
    > > [net_rx_action+362/628] [do_softirq+111/204] [do_IRQ+219/236]
    > > [ret_from_intr+0/7] [sys_ioctl+443/532] Aug 29 13:43:34 admin kernel:
    > > [system_call+51/56]
    > OK, I see what the problem is. Regular memory users are consuming memory
    > right down to the emergency reserve limit, beyond which only PF_MEMALLOC
    > users can go. Unfortunately, since atomic memory allocators can't wait,
    > they tend to fail with high frequency in this state. Duh.
    > First, there's an effective way to make these particular atomic failures
    > go away almost entirely. The atomic memory user (in this case a network
    > interrupt handler) keeps a list of pages for its private use, starting with
    > an empty list. Each time it needs a page it gets it from its private list,
    > but if that list is empty it gets it from alloc_pages, and when done with
    > it, returns it to its private list. The alloc_pages call can still fail of
    > course, but now it will only fail a few times as it expands its list up to
    > the size required for normal traffic. The effect on throughput should be
    > roughly nothing.

    Looking at the code - sg already has some private memory... but it does not
    use it in this case. (see sg_low_malloc)

    But it tries to alloc 8 (2**3) pages atomically. That it quite a bit of memory
    (32k!) that is twice more memory than the first computer I used.
    But in this case we do not see errors with lower orders, try 4*4096

    IN FILE sg.h

    #define SG_SCATTER_SZ (8 * 4096) /* PAGE_SIZE not available to user */
    /* Largest size (in bytes) a single scatter-gather list element can have.
    The value must be a power of 2 and <= (PAGE_SIZE * 32) [131072 bytes on
    i386]. The minimum value is PAGE_SIZE. If scatter-gather not supported
    by adapter then this value is the largest data block that can be
    read/written by a single scsi command. The user can find the value of
    PAGE_SIZE by calling getpagesize() defined in unistd.h . */


    Roger Larsson
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