Re: [TUHS] non-blocking IO

At around that point in time (I don't have the very _earliest_ code, to get an exact date, but the oldest traces I see [in mapalloc(), below] are from September '78), the CSR group at MIT-LCS (which were the people in LCS doing networking) was doing a lot with asynchronous I/O (when you're working below the reliable stream level, you can't just do a blocking 'read' for a packet; it pretty much has to be asynchronous). I was working in Unix V6 - we were building an experimental 1Mbit/second ring - and there was work in Multics as well. I don't think the wider Unix community heard about the Unix work, but our group regularly filed updates on our work for the 'Internet Monthly Reports', which was distributed to the whole TCP/IP experimental community. If you can find an archive of early issues (I'm too lazy to go look for one), we should be in there (although our report will alsocover the Multics TCP/IP work, and maybe some other stuff too). There were two main generations of code; I don't recall the second one well, and I'm too lazy to go look, but I can tell you off the top of my head a bit about how the first one worked. Open/read/write all looked standard to the user in the process (the latter two were oriented to packets, a bit like raw disks being blocks); multiple operations could be queued in each direction. (There was only one user allowed at a time for the network device; no input demultiplexing.) Whenever an I/O operation completed, the process was sent a signal. Since the read/write call had long since returned, it had to do a getty() to get info about that operation - the size of the packet, error indications, etc. One complication was that for a variety of reasons (we wanted to avoid having to copy data, and the interface did not have packet buffers) we did DMA directly to/from the user's memory; this meant the process has to be locked in place while I/O was pending. (I didn't realize it at the time, but we dodged a bullet there; a comment in xalloc(), which I only absorbed recently, explains the problem. More here: https://gunkies.org/wiki/UNIX_V6_internals#exec()_and_pure-text_images if anyone wants the gory details.) That all (the queing, signals for I/O completion, locking the process to a fixed location in memory while it continued to run) etc all worked well, as I recall (although I guess it couldn't do an sbrk() while locked), but one complication was the UNIBUS map on the -11/70. The DSSR/RTS group at LCS wanted to have a ring interface, but their machine was a /70 (ours, the one the driver was initially done on/for, was a /40), so with DMA we had to use the UNIBUS map. The stock V6 code had mapalloc(), (and mapfree(), both called on all DMA operations), but... it allocated the whole map to whatever I/O operation asked for the map. Clearly, if you're about to start a network input operation, and wait a packet to show up, you don't want the disk controller to have to sit and wait for for a packet to show up so _it_ can have the map. Luckily, mapalloc() was called with a pointer to the buffer header (which had all the info about the xfer), so I added a 'ubmap' array, and called the existing malloc() on it, to allocate only a big enough chunk of the UNIBUS map for the I/O operation defined by the header. Since there was 248KB of map space, and the largest single DMA transfer possible in V6 was about 64KB (maybe a little more, for a max-sized process with its 'user' block), there was never a problem with contention for the map, and we didn't have to touch any of the other drivers at all. That was dandy, and only a couple of lines of extra code, but I somehow made a math error in my changes, and as I recall I had to debug it with a printf() in mapalloc(). I was not popular that day! Luckily, the error was quickly obvious, a fix was applied, and we were on our way. Noel