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Michael K. Edwards a écrit : > On 3/8/07, Eric Dumazet <dada1@cosmosbay.com> wrote: >> Absolutely not. We dont want to slow down kernel 'just in case a fool >> might >> want to do crazy things' > > Actually, I think it would make the kernel (negligibly) faster to bump > f_pos before the vfs_write() call. Unless fget_light sets fput_needed > or the write doesn't complete cleanly, you won't have to touch the > file table entry again after vfs_write() returns. You can adjust > vfs_write to grab f_dentry out of the file before going into > do_sync_write. do_sync_write is done with the struct file before it > goes into the aio_write() loop. Result: you probably save at least an > L1 cache miss, unless the aio_write loop is so frugal with L1 cache > that it doesn't manage to evict the struct file. > > Patch to follow. Dont even try, you *cannot* do that, without breaking the standards, or without a performance drop. The only safe way would be to lock the file during the whole read()/write() syscall, and we dont want this (this would be more expensive than current) Dont forget 'file' may be some sockets/tty/whatever, not a regular file. Standards are saying : If an error occurs, file pointer remains unchanged. You cannot know for sure how many bytes will be written, since write() can returns a count that is different than buflen. So you cannot update fpos before calling vfs_write() About your L1 'miss', dont forget that multi-threaded apps are going to atomic_dec_and_test(&file->f_count) anyway when fput() is done at the end of syscall. And you were concerned about multi-threaded apps, didnt you ? - 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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