Re: Possible abuse of dpkg-deb -z9 for xz compressed binary packages
On 2014-09-03 13:10, Changwoo Ryu wrote:
> 2014-09-03 17:04 GMT+09:00 Simon McVittie <smcv@debian.org>:
>> On 02/09/14 21:17, Changwoo Ryu wrote:
>>> For fonts-nanum, the default is ~300 KiB 3.5% larger than -9e. And -9e
>>> is not better than -8e.
>>
>> I don't think anyone is arguing that higher compression settings don't
>> produce better compression ratios. However:
>>
>> Preset DictSize CompCPU CompMem DecMem
>> ...
>> -6 8 MiB 6 94 MiB 9 MiB <-
>> -7 16 MiB 6 186 MiB 17 MiB
>> -8 32 MiB 6 370 MiB 33 MiB <-
>> -9 64 MiB 6 674 MiB 65 MiB
>>
>> ... it's about cost/benefit. If we can save 300 KiB of compressed size,
>> but the cost is to more than triple the required memory to decompress
>> (from 9 MiB to 33 MiB), is that actually a worthwhile trade-off?
That is the key question, and I believe considering the worst possible
cost -- a package that cannot be unpacked, as in #757740 -- the
trade-off is not worth it.
> I think yes. The cost is 24 MiB extra memory on installation, and
> benefits are bandwidth and mirror size saving of big packages.
I think you are overestimating the benefits, and underestimating the costs.
Benefits: from the numbers posted in this thread, the size savings
compared to the default compression level for some sample packages are
somewhere around 3% to 4%. I claim that *practical* benefits from this
saving are insignificant to minor at best; counter-examples to this
claim are welcome.
Costs: The difference, to the default setting, in memory requirement may
be only 24MiB, but you forget that with this increase, you're definitely
crossing the 64MiB barrier. That is a common size for the amount of
physical RAM in embedded devices, and I believe that barrier should be
tested only for good reasons, otherwise stuff like #757740 happens.
And finally, the cost of deviating from a default setting itself, and
the implications for when that default changes, should be considered.
>> The d-i manual for wheezy on armel currently says that the bare minimum
>> RAM for wheezy is 31 MiB, the minimal recommended RAM is 64 MiB, and the
>> recommended RAM is 256 MiB or more. I'm sure those will increase
>> somewhat for jessie, but on a system with that sort of spec, packages
>> that need up to 65 MiB of RAM+swap to decompress (in addition to
>> whatever is needed for the kernel, and for the machine's actual
>> purpose!) seem rather greedy.
>
> I can't imagine any 31 MiB machine which needs to render megabytes of
> Truetype fonts.
Fair enough.
> I think we can assume usual desktop machines for font packages.
> According to the d-i manual, wheezy's minimum RAM "for desktop" is 128
> MiB, and 512 MiB is recommended. For jessie, the minimum is 256 MiB
> and 1 GiB is recommended.
OK, but to me, that assumption seems contradictory to the motive of
reducing package size. If we are assuming a sufficiently spec'ed system,
say where at least 512MiB are the norm, then the cost might be
tolerable, but at the same time, the 3% to 4% savings in package size
will hardly benefit you.
> And these requirement/recommendation are far behind of the current
> computer market.
For desktop and laptop computers, yes. However, embedded devices, which
are becoming increasingly popular, are often memory-constrained.
Christian
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