On Mon, Apr 9, 2018 at 4:58 AM, Henri Sivonen <hsivo...@hsivonen.fi> wrote:
> My understanding is that under some "huge" size, jemalloc returns > allocations from particularly-sized buckets. > The mozjemalloc source has a decent ascii-art table [1]. > This makes me expect that realloc() between bucket sizes is going to > always copy the data instead of just adjusting allocated metadata, > because to do otherwise would mess up the bucketing. > Sure, but not all reallocs are between bucket sizes. You can realloc from 1KB to 1.99KB and end up in the same bucket. > Is this so? Specifically, is it actually useful that nsStringBuffer > uses realloc() as opposed to malloc(), memcpy() with actually > semantically filled amount and free()? > > Upon superficial code reading, it seems to me that currently changing > the capacity of an nsA[C]STring might uselessly use realloc to copy > data that's not semantically live data from the string's point of view > and wouldn't really need to be preserved. Have I actually discovered > useless copying or am I misunderstanding? > In this case I think you're right. In the string code we use a doubling strategy up to 8MiB so they'll always be in a new bucket/chunk. After 8MiB we grow by 1.125 [2], but always round up to the nearest MiB. Our HugeRealloc logic always makes a new allocation if the difference is greater than or equal to 1MiB [3] so that's always going to get hit. I should note that on OSX we use some sort of 'pages_copy' when the realloc is large enough, this is probably more efficient than memcpy. -e [1] https://searchfox.org/mozilla-central/rev/7ccb618f45a1398e31a086a009f87c8fd3a790b6/memory/build/mozjemalloc.cpp#59-88 [2] https://searchfox.org/mozilla-central/rev/7ccb618f45a1398e31a086a009f87c8fd3a790b6/xpcom/string/nsTSubstring.cpp#88-119 [3] https://searchfox.org/mozilla-central/rev/7ccb618f45a1398e31a086a009f87c8fd3a790b6/memory/build/mozjemalloc.cpp#3811-3874 > _______________________________________________ dev-platform mailing list dev-platform@lists.mozilla.org https://lists.mozilla.org/listinfo/dev-platform
