Messages in this thread | | | From | "Joseph Malicki" <> | Subject | Re: sendfile | Date | Thu, 1 May 2003 11:25:28 -0400 |
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One major difference I've noticed is the interaction with the VM subsystem. When you have a large number of processes mmap'ing large files to send(), it really starts to tickle bugs and performance problems. sendfile() avoids this, only needing to use the page cache.
-joe
----- Original Message ----- From: "Mark Mielke" <mark@mark.mielke.cc> To: "Pål Halvorsen" <paalh@ifi.uio.no> Cc: "bert hubert" <ahu@ds9a.nl>; <linux-kernel@vger.kernel.org> Sent: Thursday, May 01, 2003 12:28 AM Subject: Re: sendfile
> On Thu, May 01, 2003 at 12:34:32AM +0200, Pål Halvorsen wrote: > > On Wed, 30 Apr 2003, Mark Mielke wrote: > > > To some degree, couldn't sendto() fit this description? (Assuming the > > > kernel implemented 'zero-copy' on sendto()) The benefit of sendfile() > > > is that data isn't coming from a memory location. It is coming from disk, > > > meaning that your process doesn't have to become active in order for work > > > to be done. In the case of UDP packets, you almost always want a layer on > > > top that either times the UDP packet output, or sends output in response > > > to input, mostly defeating the purpose of sendfile()... > > Maybe, but then I'll have two system calls... > > As I mentioned before, the real benefit to sendfile(), as I understand it, is > that sendfile() makes it unnecessary for the OS to fully activate the calling > process in order to do work for the calling process. Unless you can point out > some other benefit provided by sendfile(), I fail to see how you will do: > > while (1) { > send_frame_over_udp(); > sleep(); > } > > Without two system calls. Whether send_frame_over_udp() uses sendfile() as > you seem to want it to, or whether it just calls sendto(), doesn't make a > difference. Because one of your requirements is that you need to provide a > smooth feed, the primary benefit of sendfile(), that of not having to activate > your process, becomes invalid. > > I haven't done timings, or looked deeply at this part of linux-2.5.x, > however, I fail to see why the following code should not meet your > requirements: > > void *p = mmap(0, length_of_file, PROT_READ, MAP_SHARED, fd, 0); > off_t offset = 0; > > while (offset < length_of_file) > { > int packet_size = max(512, length_of_file - offset); > send(socket, &p[offset], packet_size, 0); > offset += packet_size; > usleep(packets_size * 1000000 / packets_per_second); > } > > In theory, send() should be able to provide the zero copy benefits you > are requesting. In practice, it might be a little harder, but in this > case, from my perspective, send() and sendfile() should both provide > equivalent performance. Why would sendfile() perform better than send()? > > mark > > -- > mark@mielke.cc/markm@ncf.ca/markm@nortelnetworks.com __________________________ > . . _ ._ . . .__ . . ._. .__ . . . .__ | Neighbourhood Coder > |\/| |_| |_| |/ |_ |\/| | |_ | |/ |_ | > | | | | | \ | \ |__ . | | .|. |__ |__ | \ |__ | Ottawa, Ontario, Canada > > One ring to rule them all, one ring to find them, one ring to bring them all > and in the darkness bind them... > > http://mark.mielke.cc/ > > - > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ >
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