3 ms·

> If the kernel already has the thing you’re read()’ing cached which it would do, if you have just downloaded the file. > But by reading, you’re taking those

by geocar 3y ago

> If the kernel already has the thing you’re read()’ing cached which it would do, if you have just downloaded the file. > But by reading, you’re taking those bytes and putting a copy of them in the process’s heap space i mean, you just downloaded it from the network, so unless you think mmap() can hold buffers on the network card, there's definitely going to be a copy going on. now it's downloaded, you don't need to do it again, so we're only talking the one copy here. > You can’t just say “oh I’ll read the file in and the OS will take care of it”. It doesn’t work that way. i can and do. and you've already explained swap sufficiently that i believe you know you can do exactly that also.

ninkendo 3y ago
Please keep in mind the context of the discussion. A prior poster made a claim that they can read a file into memory, and that it won’t actually use any additional memory because the kernel will “automatically take care” of it somehow. This is plainly false. You come in and say something to the effect of “but it may not have to read from disk because it’s cached”, which… has nothing to do with what was being discussed. We’re not talking about whether it incurs a disk read, we’re talking about whether it will run your system out of memory trying to load it into RAM. > i mean, you just downloaded it from the network, so unless you think mmap() can hold buffers on the network card, there's definitely going to be a copy going on. now it's downloaded, you don't need to do it again, so we're only talking the one copy here. What in god’s holy name are you blathering about? If I “just downloaded it from the network”, it’s on-disk. If I mmap() the disk contents, there’s no copy going on, it’s “mapped” to disk. If I read() the contents, which is what you said I should do, then another copy of the data is now sitting in a buffer in my process’s heap. This extra copy is now "using" memory, and if I keep doing this, I will run the system out of RAM. This is characteristically different from mmap(), where a region of memory maps to a file on-disk, and contents are faulted into memory as I read them. The reason this is an extremely important distinction, is that in the mmap scenario, the kernel is free to free the read-in pages any time it wants, and they will be faulted back again in if I try to read them again. Contrast this with using read(), which makes it so the kernel can't free the pages, because they're buffers in my process's heap, and are not considered file-backed from the kernel's perspective. > i can and do. and you've already explained swap sufficiently that i believe you know you can do exactly that also. Swap is disabled on my system. Even if it wasn’t, I’d only have so much of it. Even if I had a ton of it, read()’ing 100GB of data and relying on swap to save me is going to grind the rest of the system to a halt as the kernel tries to make room for it (because the data is in my heap, and thus isn’t file-backed, so the kernel can’t just free the pages and read them back from the file I read it from.) read() is not mmap(). Please don’t conflate them.
- SoftTalker 3y ago
  > Swap is disabled on my system. Yep I do the same. If I have a server with hundreds of GB or even TB of RAM (not uncommon these days) I'm not setting up swap. If you're exhausting that much RAM, swap is only going to delay the inevitable. Fix your program.
- geocar 3y ago
  > A prior poster made a claim that they can read a file into memory, and that it won’t actually use any additional memory because the kernel will “automatically take care” of it somehow. This is plainly false. Nobody made that claim except in your head. Why don't you read it now: Just curious: why would you not load the entire dataset "into memory" ("into memory" from a Python perspective)? Look carefully: There's no mention of the word file. For all you or I know the programmer is imagining something like this: >>> data=loaddata("https://...") Or perhaps it's an S3 bucket. There is no file, only the data set. That's more or less exactly what I do. On an average desktop/server system the OS would automatically take care of putting whatever fits in RAM and the rest on disk. You know exactly this what is meant by swap: We just confirmed that. And you know it is enabled on every average desktop server system, because you > Swap is disabled on my system are the sort of person who disables the average configuration! Can you not see you aren't arguing with anything but your own fantasies? > If I “just downloaded it from the network”, it’s on-disk. That's nonsense. It's in ram. That's the block cache you were just talking about. > If I mmap() the disk contents, there’s no copy going on, it’s “mapped” to disk Every word of that is nonsense. The disk is attached to a serial bus. Even if you're using fancy nvme "disks" there's a queue that operates in a (reasonably) serial fashion. The reason mmap() is referred to zero-copy is because it can reuse the block cache if it has been recently downloaded -- but if the data is paged out, there is absolutely a copy and it's more expensive than just read() by a long way. > Even if it wasn’t, I’d only have so much of it. Even if I had a ton of it, read()’ing 100GB of data and relying on swap to save me is going to grind the rest of the system to a halt as the kernel tries to make room for it You only have so much storage, this is life, but I can tell you as someone who does operate a 1tb of ram machine that downloads 300gb of logfiles every day, read() and write() work just fine -- I just can't speak for python (or why python people don't do it) because i don't like python.

by geocar 3y ago

ninkendo 3y ago

Please keep in mind the context of the discussion. A prior poster made a claim that they can read a file into memory, and that it won’t actually use any additional memory because the kernel will “automatically take care” of it somehow. This is plainly false. You come in and say something to the effect of “but it may not have to read from disk because it’s cached”, which… has nothing to do with what was being discussed. We’re not talking about whether it incurs a disk read, we’re talking about whether it will run your system out of memory trying to load it into RAM. > i mean, you just downloaded it from the network, so unless you think mmap() can hold buffers on the network card, there's definitely going to be a copy going on. now it's downloaded, you don't need to do it again, so we're only talking the one copy here. What in god’s holy name are you blathering about? If I “just downloaded it from the network”, it’s on-disk. If I mmap() the disk contents, there’s no copy going on, it’s “mapped” to disk. If I read() the contents, which is what you said I should do, then another copy of the data is now sitting in a buffer in my process’s heap. This extra copy is now "using" memory, and if I keep doing this, I will run the system out of RAM. This is characteristically different from mmap(), where a region of memory maps to a file on-disk, and contents are faulted into memory as I read them. The reason this is an extremely important distinction, is that in the mmap scenario, the kernel is free to free the read-in pages any time it wants, and they will be faulted back again in if I try to read them again. Contrast this with using read(), which makes it so the kernel can't free the pages, because they're buffers in my process's heap, and are not considered file-backed from the kernel's perspective. > i can and do. and you've already explained swap sufficiently that i believe you know you can do exactly that also. Swap is disabled on my system. Even if it wasn’t, I’d only have so much of it. Even if I had a ton of it, read()’ing 100GB of data and relying on swap to save me is going to grind the rest of the system to a halt as the kernel tries to make room for it (because the data is in my heap, and thus isn’t file-backed, so the kernel can’t just free the pages and read them back from the file I read it from.) read() is not mmap(). Please don’t conflate them.

SoftTalker 3y ago
> Swap is disabled on my system. Yep I do the same. If I have a server with hundreds of GB or even TB of RAM (not uncommon these days) I'm not setting up swap. If you're exhausting that much RAM, swap is only going to delay the inevitable. Fix your program.
geocar 3y ago
> A prior poster made a claim that they can read a file into memory, and that it won’t actually use any additional memory because the kernel will “automatically take care” of it somehow. This is plainly false. Nobody made that claim except in your head. Why don't you read it now: Just curious: why would you not load the entire dataset "into memory" ("into memory" from a Python perspective)? Look carefully: There's no mention of the word file. For all you or I know the programmer is imagining something like this: >>> data=loaddata("https://...") Or perhaps it's an S3 bucket. There is no file, only the data set. That's more or less exactly what I do. On an average desktop/server system the OS would automatically take care of putting whatever fits in RAM and the rest on disk. You know exactly this what is meant by swap: We just confirmed that. And you know it is enabled on every average desktop server system, because you > Swap is disabled on my system are the sort of person who disables the average configuration! Can you not see you aren't arguing with anything but your own fantasies? > If I “just downloaded it from the network”, it’s on-disk. That's nonsense. It's in ram. That's the block cache you were just talking about. > If I mmap() the disk contents, there’s no copy going on, it’s “mapped” to disk Every word of that is nonsense. The disk is attached to a serial bus. Even if you're using fancy nvme "disks" there's a queue that operates in a (reasonably) serial fashion. The reason mmap() is referred to zero-copy is because it can reuse the block cache if it has been recently downloaded -- but if the data is paged out, there is absolutely a copy and it's more expensive than just read() by a long way. > Even if it wasn’t, I’d only have so much of it. Even if I had a ton of it, read()’ing 100GB of data and relying on swap to save me is going to grind the rest of the system to a halt as the kernel tries to make room for it You only have so much storage, this is life, but I can tell you as someone who does operate a 1tb of ram machine that downloads 300gb of logfiles every day, read() and write() work just fine -- I just can't speak for python (or why python people don't do it) because i don't like python.