Messages in this thread | | | Date | Fri, 9 Jan 2009 23:17:44 +0100 | From | Willy Tarreau <> | Subject | Re: [PATCH] tcp: splice as many packets as possible at once |
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On Fri, Jan 09, 2009 at 11:12:09PM +0100, Eric Dumazet wrote: > Willy Tarreau a écrit : > > On Fri, Jan 09, 2009 at 10:24:00PM +0100, Willy Tarreau wrote: > >> On Fri, Jan 09, 2009 at 09:51:17PM +0100, Eric Dumazet wrote: > >> (...) > >>>> Also, in your second mail, you're saying that your change > >>>> might return more data than requested by the user. I can't > >>>> find why, could you please explain to me, as I'm still quite > >>>> ignorant in this area ? > >>> Well, I just tested various user programs and indeed got this > >>> strange result : > >>> > >>> Here I call splice() with len=1000 (0x3e8), and you can see > >>> it gives a result of 1460 at the second call. > > > > OK finally I could reproduce it and found why we have this. It's > > expected in fact. > > > > The problem when we loop in tcp_read_sock() is that tss->len is > > not decremented by the amount of bytes read, this one is done > > only in tcp_splice_read() which is outer. > > > > The solution I found was to do just like other callers, which means > > use desc->count to keep the remaining number of bytes we want to > > read. In fact, tcp_read_sock() is designed to use that one as a stop > > condition, which explains why you first had to hide it. > > > > Now with the attached patch as a replacement for my previous one, > > both issues are solved : > > - I splice 1000 bytes if I ask to do so > > - I splice as much as possible if available (typically 23 kB). > > > > My observed performances are still at the top of earlier results > > and IMHO that way of counting bytes makes sense for an actor called > > from tcp_read_sock(). > > > > diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c > > index 35bcddf..51ff3aa 100644 > > --- a/net/ipv4/tcp.c > > +++ b/net/ipv4/tcp.c > > @@ -522,8 +522,12 @@ static int tcp_splice_data_recv(read_descriptor_t *rd_desc, struct sk_buff *skb, > > unsigned int offset, size_t len) > > { > > struct tcp_splice_state *tss = rd_desc->arg.data; > > + int ret; > > > > - return skb_splice_bits(skb, offset, tss->pipe, tss->len, tss->flags); > > + ret = skb_splice_bits(skb, offset, tss->pipe, rd_desc->count, tss->flags); > > + if (ret > 0) > > + rd_desc->count -= ret; > > + return ret; > > } > > > > static int __tcp_splice_read(struct sock *sk, struct tcp_splice_state *tss) > > @@ -531,6 +535,7 @@ static int __tcp_splice_read(struct sock *sk, struct tcp_splice_state *tss) > > /* Store TCP splice context information in read_descriptor_t. */ > > read_descriptor_t rd_desc = { > > .arg.data = tss, > > + .count = tss->len, > > }; > > > > return tcp_read_sock(sk, &rd_desc, tcp_splice_data_recv); > > > > OK, I came to a different patch. Please check other tcp_read_sock() callers in tree :)
I've seen the other callers, but they all use desc->count for their own purpose. That's how I understood what it was used for :-)
I think it's better not to change the API here and use tcp_read_sock() how it's supposed to be used. Also, the less parameters to the function, the better.
However I'm OK for the !timeo before release_sock/lock_sock. I just don't know if we can put the rest of the if above or not. I don't know what changes we're supposed to collect by doing release_sock/ lock_sock before the if().
Regards, Willy
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