> From: Ric Mason [mailto:ric.masonn@xxxxxxxxx] > Subject: Re: zsmalloc limitations and related topics > > On 02/28/2013 07:24 AM, Dan Magenheimer wrote: > > Hi all -- > > > > I've been doing some experimentation on zsmalloc in preparation > > for my topic proposed for LSFMM13 and have run across some > > perplexing limitations. Those familiar with the intimate details > > of zsmalloc might be well aware of these limitations, but they > > aren't documented or immediately obvious, so I thought it would > > be worthwhile to air them publicly. I've also included some > > measurements from the experimentation and some related thoughts. > > > > (Some of the terms here are unusual and may be used inconsistently > > by different developers so a glossary of definitions of the terms > > used here is appended.) > > > > ZSMALLOC LIMITATIONS > > > > Zsmalloc is used for two zprojects: zram and the out-of-tree > > zswap. Zsmalloc can achieve high density when "full". But: > > > > 1) Zsmalloc has a worst-case density of 0.25 (one zpage per > > four pageframes). > > 2) When not full and especially when nearly-empty _after_ > > being full, density may fall below 1.0 as a result of > > fragmentation. > > What's the meaning of nearly-empty _after_ being full? Step 1: Add a few (N) pages to zsmalloc. It is "nearly empty". Step 2: Now add many more pages to zsmalloc until allocation limits are reached. It is "full". Step 3: Now remove many pages from zsmalloc until there are N pages remaining. It is now "nearly empty after being full". Fragmentation characteristics are different comparing after Step 1 and after Step 3 even though, in both cases, zsmalloc contains N pages. > > 3) Zsmalloc has a density of exactly 1.0 for any number of > > zpages with zsize >= 0.8. > > 4) Zsmalloc contains several compile-time parameters; > > the best value of these parameters may be very workload > > dependent. > > > > If density == 1.0, that means we are paying the overhead of > > compression+decompression for no space advantage. If > > density < 1.0, that means using zsmalloc is detrimental, > > resulting in worse memory pressure than if it were not used. > > > > WORKLOAD ANALYSIS > > > > These limitations emphasize that the workload used to evaluate > > zsmalloc is very important. Benchmarks that measure data > > Could you share your benchmark? In order that other guys can take > advantage of it. As Seth does, I just used "make" of a kernel. I run it on a full graphical installation of EL6. In order to ensure there is memory pressure, I limit physical memory to 1GB, and use "make -j20". > > throughput or CPU utilization are of questionable value because > > it is the _content_ of the data that is particularly relevant > > for compression. Even more precisely, it is the "entropy" > > of the data that is relevant, because the amount of > > compressibility in the data is related to the entropy: > > I.e. an entirely random pagefull of bits will compress poorly > > and a highly-regular pagefull of bits will compress well. > > Since the zprojects manage a large number of zpages, both > > the mean and distribution of zsize of the workload should > > be "representative". > > > > The workload most widely used to publish results for > > the various zprojects is a kernel-compile using "make -jN" > > where N is artificially increased to impose memory pressure. > > By adding some debug code to zswap, I was able to analyze > > this workload and found the following: > > > > 1) The average page compressed by almost a factor of six > > (mean zsize == 694, stddev == 474) > > stddev is what? Standard deviation. See: http://en.wikipedia.org/wiki/Standard_deviation -- 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/ . Don't email: <a href