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    SubjectRe: 2.6.24-rc2: Network commit causes SLUB performance regression with tbench
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    On Wednesday 14 November 2007 22:10, David Miller wrote:
    > From: Nick Piggin <nickpiggin@yahoo.com.au>
    > Date: Wed, 14 Nov 2007 09:27:39 +1100
    >
    > > OK, in vanilla kernels, the page allocator definitely shows higher
    > > in the results (than with Herbert's patch reverted).
    >
    > ...
    >
    > > I can't see that these numbers show much useful, unfortunately.
    >
    > Thanks for all of this data Nick.
    >
    > So the thing that's being effected here in TCP is
    > net/ipv4/tcp.c:select_size(), specifically the else branch:
    >
    > int tmp = tp->mss_cache;
    > ...
    > else {
    > int pgbreak = SKB_MAX_HEAD(MAX_TCP_HEADER);
    >
    > if (tmp >= pgbreak &&
    > tmp <= pgbreak + (MAX_SKB_FRAGS - 1) * PAGE_SIZE)
    > tmp = pgbreak;
    > }
    >
    > This is deciding, in 'tmp', how much linear sk_buff space to
    > allocate. 'tmp' is initially set to the path MSS, which
    > for loopback is 16K - the space necessary for packet headers.
    >
    > The SKB_MAX_HEAD() value has changed as a result of Herbert's
    > bug fix. I suspect this 'if' test is passing both with and
    > without the patch.
    >
    > But pgbreak is now smaller, and thus the skb->data linear
    > data area size we choose to use is smaller as well.

    OK, that makes sense. BTW, are you taking advantage of kmalloc's
    "quantization" into slabs WRT the linear data area? I wonder if
    it would be at all useful...


    > You can test if this is precisely what is causing the performance
    > regression by using the old calculation just here in select_size().
    >
    > Add something like this local to net/ipv4/tcp.c:
    >
    > #define OLD_SKB_WITH_OVERHEAD(X) \
    > (((X) - sizeof(struct skb_shared_info)) & \
    > ~(SMP_CACHE_BYTES - 1))
    > #define OLD_SKB_MAX_ORDER(X, ORDER) \
    > OLD_SKB_WITH_OVERHEAD((PAGE_SIZE << (ORDER)) - (X))
    > #define OLD_SKB_MAX_HEAD(X) (OLD_SKB_MAX_ORDER((X), 0))
    >
    > And then use OLD_SKB_MAX_HEAD() in select_size().

    That brings performance back up!

    I wonder why it isn't causing a problem for SLAB...
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