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    SubjectRe: 32GB SSD on USB1.1 P3/700 == ___HELL___ (2.6.34-rc3)
    On Thu, Apr 15, 2010 at 12:32:50PM +0800, KOSAKI Motohiro wrote:
    > > On Thu, Apr 15, 2010 at 11:31:52AM +0800, KOSAKI Motohiro wrote:
    > > > > > Many applications (this one and below) are stuck in
    > > > > > wait_on_page_writeback(). I guess this is why "heavy write to
    > > > > > irrelevant partition stalls the whole system". They are stuck on page
    > > > > > allocation. Your 512MB system memory is a bit tight, so reclaim
    > > > > > pressure is a bit high, which triggers the wait-on-writeback logic.
    > > > >
    > > > > I wonder if this hacking patch may help.
    > > > >
    > > > > When creating 300MB dirty file with dd, it is creating continuous
    > > > > region of hard-to-reclaim pages in the LRU list. priority can easily
    > > > > go low when irrelevant applications' direct reclaim run into these
    > > > > regions..
    > > >
    > > > Sorry I'm confused not. can you please tell us more detail explanation?
    > > > Why did lumpy reclaim cause OOM? lumpy reclaim might cause
    > > > direct reclaim slow down. but IIUC it's not cause OOM because OOM is
    > > > only occur when priority-0 reclaim failure.
    > >
    > > No I'm not talking OOM. Nor lumpy reclaim.
    > >
    > > I mean the direct reclaim can get stuck for long time, when we do
    > > wait_on_page_writeback() on lumpy_reclaim=1.
    > >
    > > > IO get stcking also prevent priority reach to 0.
    > >
    > > Sure. But we can wait for IO a bit later -- after scanning 1/64 LRU
    > > (the below patch) instead of the current 1/1024.
    > >
    > > In Andreas' case, 512MB/1024 = 512KB, this is way too low comparing to
    > > the 22MB writeback pages. There can easily be a continuous range of
    > > 512KB dirty/writeback pages in the LRU, which will trigger the wait
    > > logic.
    >
    > In my feeling from your explanation, we need auto adjustment mechanism
    > instead change default value for special machine. no?

    You mean the dumb DEF_PRIORITY/2 may be too large for a 1TB memory box?

    However for such boxes, whether it be DEF_PRIORITY-2 or DEF_PRIORITY/2
    shall be irrelevant: it's trivial anyway to reclaim an order-1 or
    order-2 page. In other word, lumpy_reclaim will hardly go 1. Do you
    think so?

    Thanks,
    Fengguang


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