> From: Rik van Riel [mailto:riel@xxxxxxxxxx] > Subject: Re: Followup: [PATCH -mm] make swapin readahead skip over holes > > On 04/16/2012 02:34 PM, Dan Magenheimer wrote: > > Hi Rik -- > > > > For values of N=24 and N=28, your patch made the workload > > run 4-9% percent faster. For N=16 and N=20, it was 5-10% > > slower. And for N=36 and N=40, it was 30%-40% slower! > > > > Is this expected? Since the swap "disk" is a partition > > on the one active drive, maybe the advantage is lost due > > to contention? > > There are several things going on here: > > 1) you are running a workload that thrashes > > 2) the speed at which data is swapped in is increased > with this patch > > 3) with only 1GB memory, the inactive anon list is > the same size as the active anon list > > 4) the above points combined mean that less of the > working set could be in memory at once > > One solution may be to decrease the swap cluster for > small systems, when they are thrashing. > > On the other hand, for most systems swap is very much > a special circumstance, and you want to focus on quickly > moving excess stuff into swap, and moving it back into > memory when needed. Hmmm... as I look at this patch more, I think I get a picture of what's going on and I'm still concerned. Please correct me if I am misunderstanding: What the patch does is increase the average size of a "cluster" of sequential pages brought in per "read" from the swap device. As a result there are more pages brought back into memory "speculatively" because it is presumably cheaper to bring in more pages per disk seek, even if it results in a lower "swapcache hit rate". In effect, you've done the equivalent of increasing the default swap cluster size (on average). If the above is wrong, please cut here and ignore the following. :-) But in case it is right (or close enough), let me continue... In other words, you are both presuming a "swap workload" that is more sequential than random for which this patch improves performance, and assuming a "swap device" for which the cost of a seek is high enough to overcome the costs of filling the swap cache with pages that won't be used. While it is easy to write a simple test/benchmark that swaps a lot (and we probably all have similar test code that writes data into a huge bigger-than-RAM array and then reads it back), such a test/benchmark is usually sequential, so one would assume most swap testing is done with a sequential-favoring workload. The kernbench workload apparently exercises swap quite a bit more randomly and your patch makes it run slower for low and high levels of swapping, while faster for moderate swapping. I also suspect (without proof) that the patch will result in lower performance on non-rotating devices, such as SSDs. (Sure one can change the swap cluster size to 1, but how many users or even sysadmins know such a thing even exists... so the default is important.) I'm no I/O expert, but I suspect if one of the Linux I/O developers proposed a patch that unilaterally made all sequential I/O faster and all random I/O slower, it would get torn to pieces. I'm certainly not trying to tear your patch to pieces, just trying to evaluate it. Hope that's OK. Thanks, Dan -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href