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> The great advantage of letting the kernel control caching is that great effort has been invested by the kernel developers over many decades into tuning the al

> The great advantage of letting the kernel control caching is that great effort has been invested by the kernel developers over many decades into tuning the algorithms used by the cache. Some other advantages: The kernel has a global view of what is going on with all the different applications running on the system, whereas your application only knows about itself. The cache can be shared amongst different applications. You can restart applications and the cache will stay warm.

dom0 9y ago
> The disadvantage, however, is that these algorithms are general-purpose and not tuned to the application. Another big disadvantage is that the page cache is only useful for caching data of actual files and nothing else.
- hoytech 9y ago
  Do you mean you want to cache things that can't be serialised to memory pages, or you don't want the memory pages to be backed by files? In the context of this article, many databases use the page cache to cache their data-structures, either in combination with private pages (ie postgres) or entirely (ie LMDB).
  - dom0 9y ago
    The latter case; and allocating anonymous memory is not caching.
    - hoytech 9y ago
      Oh I see, I thought you might be referring to how the page cache (obviously) can't cache things like sockets. Some applications do use the page cache for non-caching purposes though, for instance IPC, or ensuring their working-data will survive an application restart. The memory is still associated with files naturally, but this is often regarded as a feature, not a problem. Some applications use files on tmpfs so that their page cache data is backed by swap (or nothing), the same as anonymous pages.
pebers 9y ago
All true in general, I imagine probably less relevant for Scylla though which is presumably tuned for running in a more dedicated setup.
- hoytech 9y ago
  Yes, good point, I was speaking in general terms. This article shows they've put a lot of thought into it and argues convincingly that bypassing the page cache makes sense for their system.
diroussel 9y ago
For servers running big data tools, like the one described, 99.99% of the IO is going to be from the application, so you want the IO algorithms tuned to that. If there is some inefficiency in the remaining 0.01% then too bad.

dom0 9y ago

> The disadvantage, however, is that these algorithms are general-purpose and not tuned to the application. Another big disadvantage is that the page cache is only useful for caching data of actual files and nothing else.

hoytech 9y ago
Do you mean you want to cache things that can't be serialised to memory pages, or you don't want the memory pages to be backed by files? In the context of this article, many databases use the page cache to cache their data-structures, either in combination with private pages (ie postgres) or entirely (ie LMDB).
- dom0 9y ago
  The latter case; and allocating anonymous memory is not caching.
  - hoytech 9y ago
    Oh I see, I thought you might be referring to how the page cache (obviously) can't cache things like sockets. Some applications do use the page cache for non-caching purposes though, for instance IPC, or ensuring their working-data will survive an application restart. The memory is still associated with files naturally, but this is often regarded as a feature, not a problem. Some applications use files on tmpfs so that their page cache data is backed by swap (or nothing), the same as anonymous pages.

pebers 9y ago

All true in general, I imagine probably less relevant for Scylla though which is presumably tuned for running in a more dedicated setup.

hoytech 9y ago
Yes, good point, I was speaking in general terms. This article shows they've put a lot of thought into it and argues convincingly that bypassing the page cache makes sense for their system.

diroussel 9y ago

For servers running big data tools, like the one described, 99.99% of the IO is going to be from the application, so you want the IO algorithms tuned to that. If there is some inefficiency in the remaining 0.01% then too bad.