21.08.2015, 21:17, "Linus Torvalds" <[email protected]>: > On Fri, Aug 21, 2015 at 10:12 AM, Roman Gushchin <[email protected]> wrote: >> There are devices, which require custom readahead limit. >> For instance, for RAIDs it's calculated as number of devices >> multiplied by chunk size times 2. > > So afaik, the default read-ahead size is 128kB, which is actually > smaller than the old 512-page limit. > > Which means that you probably changed "ra_pages" somehow. Is it some > system tool that does that automatically, and if so based on what, > exactly?
It's just a raid driver. For instance, drivers/ms/raid5.c:6898 . On my setup I got unexpectedly even slight perfomance increase over O_DIRECT case and over old memory-based readahead limit, as you can see from numbers in the commit message (1.2GB/s vs 1.1 GB/s). So, I like an idea to delegate the readahead limit calculation to the underlying i/o level. > I'm also slightly worried about the fact that now the max read-ahead > may actually be zero, For "normal" readahead nothing changes. Only readahead syscall and madvise(MADV_WILL_NEED) cases are affected. I think, it's ok to do nothing, if readahead was deliberately disabled. > and/or basically infinite (there's a ioctl to > set it that only tests that it's not negative). Does everything react > ok to that? It's an open question, if we have to add some checks to avoid miss-configuration. In any case, we can check the limit on setting rather then adjust them dynamically. -- Roman -- 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/
