Re: Requirement: swap = RAM x 2.5 ??
On Wed, Jun 06, 2001 at 12:42:03PM -0600, Eric W. Biederman wrote: > Jeff Garzik <[EMAIL PROTECTED]> writes: > > I'm sorry but this is a regression, plain and simple. > > > > Previous versons of Linux have worked great on diskless workstations > > with NO swap. > > > > Swap is "extra space to be used if we have it" and nothing else. > > Given the slow speed of disks to use them efficiently when you are using > swap some additional rules apply. > > In the worse case when swapping is being used you get: > Virtual Memory = RAM + (swap - RAM). > > That cannot be improved. You can increase your likely hood that that > case won't come up, but that is a different matter entirely. I believe you are taking the right approach to the problem, which is not to complain about that we need 2*RAM, but to try and figure out _why_ we need 2*RAM. As far as I can make out, any pages that at one time got swapped out, will remain in swap. It is even there when there are no more references to the page, but will be reclaimed lazily (i.e. when we need to swap something new out). I'm assuming the reason we need SWAP > RAM is because once swap is filled only the subset of VM users that occupy this space are candidates for further swapping. I'm assuming this probably has a significant impact on long-running processes that have more chance of being pushed into swap at some point. The advantage of this is that when we need to remove a clean page that is already in swap we can simply discard the copy in ram, paying only a swap-in penalty. Dirty pages will have to be re-written, but we don't need to find a place to put them, swap is already reserved. If we wanted reclaim swap pages that were swapped into ram, we need to find a place to swap to, swap the page out, and eventually swap it back in. Obviously a lot more expensive. However, we must have pushed the page into swap because it was not 'pageable'. i.e. it got dirtied, and there is no underlying file to write it back to, shm, private mmap, or dirty heap. So there is infact a high likelyhood that the page will not be clean when we have to swap it out again. Now if we would reclaim not just dead swap pages, but also pages that have been swapped in but are dirtied, the 'additional cost' only involves finding a place to swap to. The nice thing is that with fewer used swap pages as a result of agressive reclaiming of swapped but in the mean time swapped back in and dirtied pages it should become a lot easier to find a free spot (until we are really overcommitted). I don't know how feasable it is to tell from a given swap page, whether there is a dirtied copy present in ram, but we could drop the swap reference when the copy in ram is modified, turning the swap page into a dead page and letting the regular reclamation pick it up. Jan - 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/
Re: Requirement: swap = RAM x 2.5 ??
Jeff Garzik <[EMAIL PROTECTED]> writes: > I'm sorry but this is a regression, plain and simple. > > Previous versons of Linux have worked great on diskless workstations > with NO swap. > > Swap is "extra space to be used if we have it" and nothing else. Given the slow speed of disks to use them efficiently when you are using swap some additional rules apply. In the worse case when swapping is being used you get: Virtual Memory = RAM + (swap - RAM). That cannot be improved. You can increase your likely hood that that case won't come up, but that is a different matter entirely. I suspect in practice that we are suffering more from lazy reclamation of swap pages than from a more aggressive swap cache. Eric - 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/
Re: Requirement: swap = RAM x 2.5 ??
In article <[EMAIL PROTECTED]>, Richard Gooch <[EMAIL PROTECTED]> wrote: > > Swap is "extra space to be used if we have it" and nothing else. > > Sure. But Linux still works without swap. It's just that if you *do* > have swap, it works best with 2* RAM. There is a large difference between saying Linux works best with 2*RAM, and Linux requires 2*RAM. - 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/
Re: Requirement: swap = RAM x 2.5 ??
On Wed, 6 Jun 2001, Richard Gooch wrote: > Jeff Garzik writes: > > Richard Gooch wrote: > > > > > > Jeff Garzik writes: > > > > > > > > I'm sorry but this is a regression, plain and simple. > > > > > > > > Previous versons of Linux have worked great on diskless workstations > > > > with NO swap. > > > > > > > > Swap is "extra space to be used if we have it" and nothing else. > > > > > > Sure. But Linux still works without swap. It's just that if you *do* > > > have swap, it works best with 2* RAM. > > > > Yes, but that's not the point of the discussion. Currently 2*RAM is > > more of a requirement than a recommendation. > > Um, do you mean "2*RAM is required, always", or "2*RAM or more swap is > required if swap != 0"? When Rik starts to reclaim unused swap (didn't he say he was going to do that?) this will instantly revert to the most respected of rules.. rules are made to be b0rken. -Mike - 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/
Re: Requirement: swap = RAM x 2.5 ??
Jeff Garzik writes: > Richard Gooch wrote: > > > > Jeff Garzik writes: > > > > > > I'm sorry but this is a regression, plain and simple. > > > > > > Previous versons of Linux have worked great on diskless workstations > > > with NO swap. > > > > > > Swap is "extra space to be used if we have it" and nothing else. > > > > Sure. But Linux still works without swap. It's just that if you *do* > > have swap, it works best with 2* RAM. > > Yes, but that's not the point of the discussion. Currently 2*RAM is > more of a requirement than a recommendation. Um, do you mean "2*RAM is required, always", or "2*RAM or more swap is required if swap != 0"? Regards, Richard Permanent: [EMAIL PROTECTED] Current: [EMAIL PROTECTED] - 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/
Re: Requirement: swap = RAM x 2.5 ??
Richard Gooch wrote: > > Jeff Garzik writes: > > > > I'm sorry but this is a regression, plain and simple. > > > > Previous versons of Linux have worked great on diskless workstations > > with NO swap. > > > > Swap is "extra space to be used if we have it" and nothing else. > > Sure. But Linux still works without swap. It's just that if you *do* > have swap, it works best with 2* RAM. Yes, but that's not the point of the discussion. Currently 2*RAM is more of a requirement than a recommendation. -- Jeff Garzik | Andre the Giant has a posse. Building 1024| MandrakeSoft | - 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/
Re: Requirement: swap = RAM x 2.5 ??
Jeff Garzik writes: > > I'm sorry but this is a regression, plain and simple. > > Previous versons of Linux have worked great on diskless workstations > with NO swap. > > Swap is "extra space to be used if we have it" and nothing else. Sure. But Linux still works without swap. It's just that if you *do* have swap, it works best with 2* RAM. Regards, Richard Permanent: [EMAIL PROTECTED] Current: [EMAIL PROTECTED] - 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/
Requirement: swap = RAM x 2.5 ??
I'm sorry but this is a regression, plain and simple. Previous versons of Linux have worked great on diskless workstations with NO swap. Swap is "extra space to be used if we have it" and nothing else. -- Jeff Garzik | Andre the Giant has a posse. Building 1024| MandrakeSoft | - 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/
