On Thu, Dec 05, 2013 at 06:45:22AM -0700, Eric Blake wrote: >On 12/04/2013 10:51 AM, Christopher Faylor wrote: > >>>>> One question, though. Assuming start is == size, then the current code >>>>> in CVS extends the fd table by only 1. If that happens often, the >>>>> current code would have to call ccalloc/memcpy/cfree a lot. Wouldn't >>>>> it in fact be better to extend always by at least NOFILE_INCR, and to >>>>> extend by (1 + start - size) only if start is > size + NOFILE_INCR? >>>>> Something like >>>>> >>>>> size_t extendby = (start >= size + NOFILE_INCR) ? 1 + start - size : >>>>> NOFILE_INCR; >>>>> > >Always increasing by a minimum of NOFILE_INCR is wrong in one case - we >should never increase beyond OPEN_MAX_MAX (currently 3200). dup2(0, >3199) should succeed (unless it fails with EMFILE due to rlimit, but we >already know that our handling of setrlimit(RLIMIT_NOFILE) is still a >bit awkward); but dup2(0, 3200) must always fail with EBADF. I think >the code in CVS is still wrong: we want to increase to the larger of the >value specified by the user or NOFILE_INCR to minimize repeated calloc, >but we also need to cap the increase to be at most OPEN_MAX_MAX >descriptors, to avoid having a table larger than what the rest of our >code base will support.
I made some more changes to CVS. Incidentally did you catch the fact that you broke how this worked in 1.7.26? You were taking a MAX of a signed and unsigned quantity so the signed quantity was promoted to a huge positive number. >Not having NOFILE_INCR free slots after a user allocation is not fatal; No one implied it was. >it means that the first allocation to a large number will not have tail >padding, but the next allocation to fd+1 will allocate NOFILE_INCR slots >rather than just one. My original idea of MAX(NOFILE_INCR, start - >size) expresses that. That wasn't Corinna's concern. My replacement code would have called calloc for every one of: dup2(0, 32); dup2(1, 33); dup2(2, 34); Obviously there are different ways to avoid this and I chose to extend the table after the "start" location. >>> That might be helpful. Tcsh, for instance, always dup's it's std >>> descriptors to the new fds 15-19. If it does so in this order, it would >>> have to call extend 5 times. >> >> dtable.h:#define NOFILE_INCR 32 >> >> It shouldn't extend in that scenario. The table starts with 32 >> elements. > >Rather, the table starts with 256 elements; which is why dup2 wouldn't >crash until dup'ing to 256 or greater before I started touching this. The table is initialized in dtable_init() with 32 elements. When it enters main, it is still 32 elements, at least according to cygheap->fdtab.size. I just checked this with gdb. cgf