On Wednesday 13 January 2016 17:27:16 Dave Chinner wrote: > > I think > > it was more than that when I first looked, so it's between 0.2% and 0.3% > > of savings in total memory, which is certainly worth discussing about, > > given the renewed interest in conserving RAM in general. If we want to > > save this memory, then doing it at the same time as the timespec64 conversion > > is the right time so we don't need to touch every file twice. > > You just uttered the key words: "If we want to save this memory" > > So let's stop conflating two different lines of development because > we only actually *need* y2038k support. > > The fact we haven't made timestamp space optimisations means > that nobody has thought it necessary or worthwhile. y2038k support > doesn't change the landscape under which we might consider the > optimisation, so we need to determine if the benefit outweighs the > cost in terms of code complexity and maintainability. > > So separate the two changes - make the y2038k change simple and > obviously correct first by changing everything to timespec64. Then it > won't get delayed by bikeshedding about an optimisation of that is > of questionable benefit. Fine with me. I think Deepa already started simplifying the series already. I agree that for 64-bit machines, there is no need to optimize that code now, since we are not regressing in terms of memory size. For 32-bit machines, we are regressing anyway, the question is whether it's by 12 or 24 bytes per inode. Let me try to estimate the worse-case scenario here: let's assume that we have 1GB of RAM (anything more on a 32-bit system gets you into trouble, and if you have less, there will be less of a problem). Filling all of system ram with small tmpfs files means a single 4K page plus 280 bytes for the minimum inode, so we need an additional 6MB or 12MB to store the extra timespec bits. Probably not too bad for a worst-case scenario, but there is also the case of storing just the inodes but no pages, and that would be worse. I've added the linux-arm and linux-mips lists to cc, to see if anyone has strong opinions on this matter. We don't have to worry about x86-32 here, because sizeof(struct timespec64) is 12 bytes there anyway, and I don't think there are any other 32-bit architectures that have large-scale deployments or additional requirements we don't already have on ARM. Arnd
