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The Local structure is 2104 bytes long and jemalloc rounds this to 4096 bytes. #551

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PaulBone opened this issue Sep 1, 2020 · 4 comments · Fixed by #552 · May be fixed by #963
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The Local structure is 2104 bytes long and jemalloc rounds this to 4096 bytes. #551

PaulBone opened this issue Sep 1, 2020 · 4 comments · Fixed by #552 · May be fixed by #963
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crossbeam-epoch

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@PaulBone
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PaulBone commented Sep 1, 2020

The Local structure here https://searchfox.org/mozilla-central/source/third_party/rust/crossbeam-epoch/src/internal.rs#287 is 2104 bytes long. When compiled into Firefox and using the jamalloc memory allocator this is rounded up to 4096 bytes (a very common practice). This wastes just under 12KB for a Firefox process that loads a simple website. It'd be nice if it could allocate 2048 bytes or less saving this memory.

Thanks.
@taiki-e
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taiki-e commented Sep 1, 2020

Local::bag field is 2056 bytes and already exceeds 2048 bytes, so there seems to be no other way to do this than reduce the size of Bag.

crossbeam/crossbeam-epoch/src/internal.rs

Lines 362 to 363 in bfb7705

/// The local bag of deferred functions.
pub(crate) bag: UnsafeCell<Bag>,

And to reduce the size of the Bag, it needs to reduce MAX_OBJECTS.

crossbeam/crossbeam-epoch/src/internal.rs

Lines 56 to 67 in bfb7705

/// Maximum number of objects a bag can contain.
#[cfg(not(feature = "sanitize"))]
const MAX_OBJECTS: usize = 64;
#[cfg(feature = "sanitize")]
const MAX_OBJECTS: usize = 4;
/// A bag of deferred functions.
pub struct Bag {
/// Stashed objects.
deferreds: [Deferred; MAX_OBJECTS],
len: usize,
}

@stjepang: Is it okay to reduce MAX_OBJECTS? Or do you think we need to think of another way?

@ghost
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ghost commented Sep 1, 2020

It should be totally fine to reduce it to 31 (if my calculation is right).

@taiki-e taiki-e mentioned this issue Sep 1, 2020
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@bors bors bot closed this as completed in ce53365 Sep 1, 2020
@PaulBone
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PaulBone commented Sep 2, 2020

Awesome thanks.

@taiki-e
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taiki-e commented Jul 22, 2022

Reopening -- #552 has been reverted in #879. (See #869 for the context.)

@taiki-e taiki-e reopened this Jul 22, 2022
@danielkeller danielkeller linked a pull request Feb 25, 2023 that will close this issue
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