id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc	launchpad_bug
1398	make docs/performance.rst more precise and accurate	zooko	warner	"In comment:13:ticket:1395, Brian wrote, concerning the [http://tahoe-lafs.org/trac/tahoe-lafs/browser/trunk/docs/performance.rst?rev=3523b50716e65575#performing-a-file-verify-on-an-a-byte-file ""Performing a file-verify on an A-byte file""]:

 to be ""N/K*S times a small multiple"". I think the multiple is currently about 2 or 3. During encryption, we hold both a plaintext share and a ciphertext share in RAM at the same time (so 2*S), then we drop the plaintext. During erasure-coding, we hold a whole S of ciphertext in memory at the same time as the N/K*S shares, then we drop the ciphertext before pushing. We also pipeline the sends a little bit, I think 10kB or 50kB per server, to get better utilization out of a non-zero-latency wire.

 Also Python's memory-management strategy interacts weirdly. Dropping the plaintext segment may not be enough: Python might not re-use that memory space for anything else right away. Although I'd expect it to de-fragment or coalesce free blocks before asking the OS for so much memory that it crashed.

Although I wonder if Brian was thinking of repair rather than verify since he talks about encrypting, which is not done in verify.

Subsequently I reviewed the document and I see a bunch of things I'm not sure are right. (Note that I myself am mostly responsible for the current state of this document.)

* [http://tahoe-lafs.org/trac/tahoe-lafs/browser/trunk/docs/performance.rst?rev=3523b50716e65575#publishing-an-a-byte-immutable-file ""Publishing an A-byte immutable file"" / ""when the file is already uploaded""]: {{{memory footprint: N/K*S}}}. Shouldn't that just be {{{memory footprint: S}}}? All it does is read each {{{S}}}-byte segment in turn and hash it. {{{K}}} and {{{N}}} shouldn't come into it. This is probably just a cut-and-paste error of think-o error on my part originally, so unless someone else knows of a better reason why I wrote that then I'm going to change it to {{{memory footprint: S}}}.

* [http://tahoe-lafs.org/trac/tahoe-lafs/browser/trunk/docs/performance.rst?rev=3523b50716e65575#publishing-an-a-byte-immutable-file ""Publishing an A-byte immutable file"" / ""when the file is not already uploaded""]: if we're going to make the {{{memory footprint}}} more precise as Brian suggests above then this one should be changed too. Also {{{network: ~N + ~A}}} should actually be {{{network: N/K*~A}}}, right?

* [http://tahoe-lafs.org/trac/tahoe-lafs/browser/trunk/docs/performance.rst?rev=3523b50716e65575#downloading-b-bytes-of-an-a-byte-immutable-file ""Downloading B bytes of an A-byte immutable file""]: {{{cpu: ~A}}}. What? The CPU usage for downloading {{{B}}} bytes of an {{{A}}}-byte immutable file is {{{~A}}}? I really hope it is actually {{{~B}}} (plus an amount of CPU logarithmic in {{{A}}} for Merkle Tree verification, but I'm not sure we should try to include that much precision in this document). Unless someone tells me I'm wrong now and was right then, I'm going to change this to {{{cpu: ~B}}}.

* [http://tahoe-lafs.org/trac/tahoe-lafs/browser/trunk/docs/performance.rst?rev=3523b50716e65575#repairing-an-a-byte-file-mutable-or-immutable ""Repairing an A-byte file""] {{{network: variable; up to around ~A}}}: surely that should say {{{network: variable; up to around N/K*A}}}"	defect	closed	major	1.9.2	documentation	1.8.2	fixed	scalability docs performance memory large	zooko