Question on Partial Checksum in TCP/IP
Hello all, I am trying to understand a little bit about the TCP path in the Linux kernel. I saw that while we were even copying the user data into kernel space, we were doing the partial checksum of the data portion alone (as the TCP header is not yet filled up) and storing it in skb->csum. Later on when we fill the header portion, we do the checksum of the header and "add" it to the already calculated checksum of the data. Here we check if the HW performs the checksum or not (skb->ip_summed == CHECKSUM_HW). I am looking into kernel ver 2.4 My question is, why is this check not performed when the data is copied initially. Instead of calling csum_and_copy_from_user, if we did a memcpy in the case when HW does the checksum, would'nt that be more efficient? Or have I overlooked something... Thanks a lot PS. Please CC responses to me also as I am not subscribed to the mailing list. Thanks Bharath - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Question on Partial Checksum in TCP/IP
Hello all, I am trying to understand a little bit about the TCP path in the Linux kernel. I saw that while we were even copying the user data into kernel space, we were doing the partial checksum of the data portion alone (as the TCP header is not yet filled up) and storing it in skb-csum. Later on when we fill the header portion, we do the checksum of the header and add it to the already calculated checksum of the data. Here we check if the HW performs the checksum or not (skb-ip_summed == CHECKSUM_HW). I am looking into kernel ver 2.4 My question is, why is this check not performed when the data is copied initially. Instead of calling csum_and_copy_from_user, if we did a memcpy in the case when HW does the checksum, would'nt that be more efficient? Or have I overlooked something... Thanks a lot PS. Please CC responses to me also as I am not subscribed to the mailing list. Thanks Bharath - To unsubscribe from this list: send the line unsubscribe linux-kernel in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
RE: Accelerated TCP/IP support from kernel
Hello all, We will most probably be looking into supporting Gig MACs and that is the reason why I was wondering if HW assist will help. We are talking of Gigs of network data (dont ask me the application!) I dont know about IPv6 support, we havent thought of that. But the point is, We can visualize a scenario where the processor is overworked and we want to relieve it off some of its load. To summarize the info I got from the mailing list, there are two conflicting views about this topic. 1. Protocol processing does not take much time compared to data processing and so the ZERO_COPY/HW_CHECKSUM feature in Linux currently is good enough. 2. The application maybe unique and heavy load that it may save the processor some bandwidth especially when several TCP connections needs to be maintained. I am mainly looking into case 2, and just trying to see the amount of work that needs to go into this area. I am asking your expert opinions and find out if this is an easy job, if so can it be done to maintain compatability with older NICs too and able to move along easily with standard linux distributions. If the kernel can provide some hooks to "branch off" to a different set of drivers at the socket level, that will do the trick I think. Is this easy? Thanks a lot Bharath -Original Message- From: David S. Miller [mailto:[EMAIL PROTECTED]] Sent: Thursday, May 24, 2001 7:08 PM To: Bharath Madhavan Cc: '[EMAIL PROTECTED]' Subject: RE: Accelerated TCP/IP support from kernel Bharath Madhavan writes: > I guess 3c905c NIC supports HW checksumming. Is this true? Yes. > In this case, do we have any benchmarking for this card > with and without ZERO_COPY (and HW checksumming). I am eager to > know by how many times did the system throughput increase? It doesn't matter with 100baseT cards, they are slow enough that even with the cpu doing the data copies the link may be easily saturated. What you will get is decreased CPU utilization. You need to go to gigabit or faster link speeds to see any real throughput improvement. Later, David S. Miller [EMAIL PROTECTED] - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
RE: Accelerated TCP/IP support from kernel
Hello all, We will most probably be looking into supporting Gig MACs and that is the reason why I was wondering if HW assist will help. We are talking of Gigs of network data (dont ask me the application!) I dont know about IPv6 support, we havent thought of that. But the point is, We can visualize a scenario where the processor is overworked and we want to relieve it off some of its load. To summarize the info I got from the mailing list, there are two conflicting views about this topic. 1. Protocol processing does not take much time compared to data processing and so the ZERO_COPY/HW_CHECKSUM feature in Linux currently is good enough. 2. The application maybe unique and heavy load that it may save the processor some bandwidth especially when several TCP connections needs to be maintained. I am mainly looking into case 2, and just trying to see the amount of work that needs to go into this area. I am asking your expert opinions and find out if this is an easy job, if so can it be done to maintain compatability with older NICs too and able to move along easily with standard linux distributions. If the kernel can provide some hooks to branch off to a different set of drivers at the socket level, that will do the trick I think. Is this easy? Thanks a lot Bharath -Original Message- From: David S. Miller [mailto:[EMAIL PROTECTED]] Sent: Thursday, May 24, 2001 7:08 PM To: Bharath Madhavan Cc: '[EMAIL PROTECTED]' Subject: RE: Accelerated TCP/IP support from kernel Bharath Madhavan writes: I guess 3c905c NIC supports HW checksumming. Is this true? Yes. In this case, do we have any benchmarking for this card with and without ZERO_COPY (and HW checksumming). I am eager to know by how many times did the system throughput increase? It doesn't matter with 100baseT cards, they are slow enough that even with the cpu doing the data copies the link may be easily saturated. What you will get is decreased CPU utilization. You need to go to gigabit or faster link speeds to see any real throughput improvement. Later, David S. Miller [EMAIL PROTECTED] - To unsubscribe from this list: send the line unsubscribe linux-kernel in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/ - To unsubscribe from this list: send the line unsubscribe linux-kernel in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Accelerated TCP/IP support from kernel
Hello all, I am looking into a scenario where we have a NIC which performs all the TCP/IP processing and basically the core CPU offloads all data from the socket level interface onwards to this NIC. Can Linux do this as of now. I saw some limited support like TCP/IP checksumming being done in HW (ex: 3c905c) and linux supports it. I also saw the ZERO_COPY feature in linux code. My question is can there be support for entire (almost) TCP/IP to be offloaded to a NIC card. I noticed some NIC cards by Alacritec and Applicom who have such NICs but it looks like the support from Linux would be quite a bit of work. Basically Linux needs to support all socket calls as it will usually but the socket interface must be intelligent enough to call different drivers(!) for those NIC cards and thus bypassing the whole of TCP/IP stack of Linux. Is this possible at all and if so, is anyone doing this??? I talked to a vendor who does this NIC and he said that they provide a library for linux which means that they have proprietery functions for using their NIC and thus this cannot be used in the standard socket interface way. That is a big limitation and not of much use. Basically, I am looking into a case where the kernel code needs to be changed to handle this Looking eagerly for some help/advice from you folks Thanks a lot Bharath - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Accelerated TCP/IP support from kernel
Hello all, I am looking into a scenario where we have a NIC which performs all the TCP/IP processing and basically the core CPU offloads all data from the socket level interface onwards to this NIC. Can Linux do this as of now. I saw some limited support like TCP/IP checksumming being done in HW (ex: 3c905c) and linux supports it. I also saw the ZERO_COPY feature in linux code. My question is can there be support for entire (almost) TCP/IP to be offloaded to a NIC card. I noticed some NIC cards by Alacritec and Applicom who have such NICs but it looks like the support from Linux would be quite a bit of work. Basically Linux needs to support all socket calls as it will usually but the socket interface must be intelligent enough to call different drivers(!) for those NIC cards and thus bypassing the whole of TCP/IP stack of Linux. Is this possible at all and if so, is anyone doing this??? I talked to a vendor who does this NIC and he said that they provide a library for linux which means that they have proprietery functions for using their NIC and thus this cannot be used in the standard socket interface way. That is a big limitation and not of much use. Basically, I am looking into a case where the kernel code needs to be changed to handle this Looking eagerly for some help/advice from you folks Thanks a lot Bharath - To unsubscribe from this list: send the line unsubscribe linux-kernel in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
Speeding up VFS using HW assist
Hello All, I will be using Linux as the OS for an embedded system. I was looking into 2.4.4 kernel code and saw the dcache implementation in VFS which is pretty neat and fast by itself. My question is, will I gain any considerable efficiency in file system access if I can move this "pathname -> inode" lookup into some proprietery HW assist mechanism and take out the dcache hashing and "cached_lookup" function. How good(bad) was it before the dcache implementation and in which release was dcache feature added (was it only after 2.2.x release). Did we get 2-3 times better performance with dcache? (if not, how much?) Can anyone suggest any other place in the file system (VFS and EXT2) where we can use any HW assist (let us say FPGA implementing search, lookup, etc.) to speed up file-system access (both for opening and read/write) Would tweaking the buffer cache and page cache sizes make a considerable effect on efficiency? Any other suggestions? Thanks a lot, Bharath - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
