On 5/12/20 11:22 PM, Michał Górny wrote: > On Tue, 2020-05-12 at 13:18 -0700, Zac Medico wrote: >> On 5/12/20 12:28 PM, Michał Górny wrote: >>> W dniu wto, 12.05.2020 o godzinie 10∶05 -0700, użytkownik Zac Medico >>> napisał: >>>> On 5/12/20 3:39 AM, Michał Górny wrote: >>>>> I'm sorry but I was asking of a more high-level implications. >>>>> >>>>> I presume that this means that more than files of more than one >>>>> package >>>>> can be merged simultaneously. However: >>>> >>>> No, an exclusive lock must be held on vardbapi._fs_lock for this. >>>> This >>>> is currently required at least to guarantee that access to the config >>>> memory file is serialized (config memory is the thing that emerge >>>> --noconfmem disables, but --noconfmem does not disable this lock). >>>> >>>> We assume that it's probably not worthwhile to try to merge files for >>>> more than one package at a time, since that would cause them to >>>> compete >>>> for IO bandwidth. >>>> >>>>> 1. Are collisions handled correctly then? i.e. if you start >>>>> installing >>>>> A, and then B, and the two packages collide will portage fail >>>>> before >>>>> starting to install any file from B? >>>> >>>> There are no guarantees here. However, the risk is minimal, since >>>> it's >>>> unlikely that a file collision of this sort would occur. file >>>> collisions >>>> are a QA problem that is generally detected and corrected log before >>>> we >>>> would encounter a collision of this sort. >>>> >>>>> 2. Are preinst/postinst phases called simultaneously or serialized? >>>> >>>> They're serialized. >>> >>> Now I'm lost here. Could you try to explain to me, without getting >>> into the deep technicalities, how parallel-install achieves better >>> speed or at doing what is non-parallel-install so slow? >> >> It allows preinst/postinst/prerm/postrm phases to run for one package >> while files are concurrently being merged or unmerged for another >> package. This makes it possible to approach saturation of IO bandwidth. > > Doesn't this imply that programs run in these phases could fail if > Portage is simultaneously replacing their files?
We've got the same potential issue with emerge --jobs, but we use the dependency graph to schedule merges such that our dependencies do not mutate underneath us. -- Thanks, Zac
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