On Fri, Jul 22, 2011 at 07:07:41PM -0500, Matt Mackall wrote: > On Sat, 2011-07-23 at 08:59 +1000, NeilBrown wrote: > > On Fri, 22 Jul 2011 18:31:58 -0400 "J. Bruce Fields" <bfields@xxxxxxxxxxxx> > > wrote: > > > > > On Fri, Jul 22, 2011 at 06:10:39PM -0400, bfields wrote: > > > > On Fri, Jul 22, 2011 at 11:47:32PM +0200, Andi Kleen wrote: > > > > > On Fri, Jul 22, 2011 at 04:11:42PM -0500, Matt Mackall wrote: > > > > > > On Fri, 2011-07-22 at 22:59 +0200, Andi Kleen wrote: > > > > > > > > Indeed. Only usefully exists on ext4 and requires extra system calls. > > > > > > > > > > > > > > Not sure what you mean? It's in stat(2), just like the timestamps. > > > > > > > > > > > > I don't see anything that looks like a version or generation number in > > > > > > either the man pages, the asm-generic/stat.h, or glibc's asm/stat.h. > > > > > > Pointer? > > > > > > > > > > Hmm you're right. I thought it was in there, but apparently not. > > > > > I think it should be added there though. We still have some unused > > > > > fields. > > > > > > > > But last I checked I thought it was only ext4 that actually incremented > > > > the i_version on IO, and even then only when given a (non-default) mount > > > > option. > > > > > > > > My notes on what needs to be done there: > > > > > > > > - collect data to determine whether turning on i_version causes > > > > any significant performance regressions. > > > > - Last I talked to him, Ted Tso recommended running > > > > Bonnie on a local disk, since it does a lot of little > > > > writes, which is somewhat of a worst case, as it will > > > > generate extra metadata updates for each write. > > > > Compare total wall-clock time, number of iops, and > > > > number of bytes (using some kind of block tracing). > > > > - If there aren't any problems, turn it on by default, and we're > > > > done. > > > > > > (Well,and talk the other filesystem implementors into doing it.) > > > > > > > But does anyone apart from NFSv4 actually *want* i_version as opposed to the > > more-generally-useful precise timestamps? > > In theory, a microsecond timestamp (ie gtod) may already not be good > enough for all applications. But i_version also doesn't allow comparing > across files. > > > If not, we probably should tell NFSv4 to use timestamps and focus on making > > them work well. > > ?? > > > > The timestamp used doesn't need to update ever nanosecond. I think if it > > were just updated on every userspace->kernel transition (or effective > > equivalents inside kernel threads) that would be enough capture all > > causality. I wonder how that would be achieved.. I wonder if RCU machinery > > could help - doesn't it keep track of when threads schedule ... or something? > > Sort of. > > Some observations: > > - we only need to go to higher resolution when two events happen in the > same time quantum > - this applies at both the level of seconds and jiffies > - if the only file touched in a given quantum gets touched ago, we don't > need to update its timestamp if stat wasn't also called on it in this > quantum > - we never need to use a higher resolution than the global > min(s_time_gran) Right, so there was a rough algorithm hashed out somewhere around here: http://thread.gmane.org/gmane.linux.kernel/1022866/focus=1024624 that depended on those observations. NFS presents a worst-case as the standard NFSv3 read and write operations include timestamps in the result. So every single IO comes with a stat. So either you have a clock good enough to give a distinct timestamp for all of those, or you fall back on a global counter that ends up serializing all IO. I think. I admit I'm not sure I understand your proposal below. --b. > > > For instance, if a machine is idle, except for writing to a single file > once a second, 1s resolution suffices. > > If a machine is idle, except for writing to the same file 1000 times per > second, and no one is watching it, 1s still suffices (inode is dirtied > once per second). > > Any time two files are touched in the same second, the second one (and > later files) needs jiffies resolution. Similarly, any time two files are > touched in the same jiffy, the second one should use gtod(). > > The global status bits needed to track this could be managed fairly > efficiently with cmpxchg. > > (Arguably, we should supply > 1s resolution whether they're strictly > needed or not on filesystems with nanosecond support, so that people > casually inspecting timestamps don't wonder where their nanoseconds > went.) > > -- > Mathematics is the supreme nostalgia of our time. > > -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
