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xen-devel
Re: [Xen-devel] huge tcp performance-regression on pvops - kernel (+Solu
On Tue, Oct 27, 2009 at 12:37:25AM +0100, Ronny.Hegewald@xxxxxxxxx wrote:
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> repeat-x scroll center bottom; } .correct {} .unknown {} .ignored
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> <p>Setup: pvops dom0 kernel 2.6.31.4 from Jeremys git-repository from
> 2009-10-18<span></span></p><p id="__paragraph__1256597098000" style=""><span
> style=""><br></span></p>Problem: Very slow network-performance (3-10 kbs)
> when tcp-packets are used (noticed when using scp, samba, nfs over tcp)<p
> id="__paragraph__1256597073000" style=""><br></p><p
> id="__paragraph__1256597073000" style="">But this occurs only in the
> following situations:<br></p><p id="__paragraph__1256597073000"
> style=""> <span></span></p><p
> id="__paragraph__1256597073000" style="">1.) domU to domU on same PC (domUs
> are paravirtualized linux-kernels)<br></p><p id="__paragraph__1256597073000"
> style="">2.) domU to another PC that doesn't use<span id="misspelled-have"
> class="misspelled" name="misspelled-have"></span> pvops-kernel (packets sent
> from another PC to domU works fine)<br></p><br>domU to dom0 and the opposite
> way works without performance-regression.<br><br><p>Reason: bigger
> tcp-packets get dropped from the domU the tcp-packets are sent from (netstat
> -s in domU shows many retransmitted tcp-segments)</p><p><br></p><p>tcpdump
> shows that the bigger packets leave the vif from the domU they were sent
> from, but never arrive the vif from the domU they are sent
> to.<br><span></span></p><br><span style=""></span><p>This is caused by this
> lines in drivers/xen/netback.c at line 1325 :<span></span></p><p
> id="__paragraph__1256597423000" style=""><br><span style=""></span></p><p
> id="__paragraph__1256597423000"
> style="">
> if (skb->data_len < skb_shinfo(skb)->gso_size) {
> </p><div
> class="pre">
> skb_shinfo(skb)->gso_size = 0;</div>
> <div
> class="pre">
> skb_shinfo(skb)->gso_type = 0;</div>
> <div
> class="pre">
> }</div><br><p id="__paragraph__1256597121000" style="">These lines were
> reverted from the linux-2.6.18-xen mercurial repository <br></p><p
> id="__paragraph__1256597121000" style=""><br></p><p
> id="__paragraph__1256597121000" style="">on 2009-01-13 in changeset 774:
> <a
> href="http://xenbits.xensource.com/linux-2.6.18-xen.hg?rev/107e10e0e07c";>107e10e0e07c:
> netfront/back: do not mark packets of length < MSS as GSO</a></p><p
> id="__paragraph__1256598462000" style=""><br><span style=""></span></p><p
> id="__paragraph__1256598462000" style=""><span style="">I used the patch on
> the above mentioned pvops tree and the problem was
> gone.</span><span></span></p><p id="__paragraph__1256600217000"
> style=""><br><span style=""></span></p><p id="__paragraph__1256600217000"
> style=""><span style="">I never noticed such problems on the 2.6.18-xen
> kernel or the forward-ported xen-kernel 2.6.31.4 (from </span>Andrew
> Lyon)</p></body></>
>
Kudos for discovering this. Did you verify whether the domU has the
corresponding
patch:
diff -r 28acedb66302 -r 107e10e0e07c drivers/xen/netfront/netfront.c
--- a/drivers/xen/netfront/netfront.c Wed Jan 07 12:21:54 2009 +0900
+++ b/drivers/xen/netfront/netfront.c Tue Jan 13 15:17:54 2009 +0000
@@ -1439,6 +1439,14 @@
np->stats.rx_packets++;
np->stats.rx_bytes += skb->len;
+#if HAVE_TSO
+ if (skb->data_len < skb_shinfo(skb)->gso_size) {
+ skb_shinfo(skb)->gso_size = 0;
+#if HAVE_GSO
+ skb_shinfo(skb)->gso_type = 0;
+#endif
+ }
+#endif
__skb_queue_tail(&rxq, skb);
np->rx.rsp_cons = ++i;
Looking at the PV_OPs kernel it doesn't look to be there, but I was
wondering if the domU you are using has it?
Ian,
Would it make sense to remove this changeset if most of the DomU's
don't have this fix?
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